Environ. Sci. Technol, 1907, 27, 1194-1201
Human Exposure to Volatile Organic Compounds in Household Tap Water: The Indoor Inhalation Pathway Thomas E. McKone Environmental Sciences Division, Lawrence Livermore National Laboratory, Livermore, California 94550 This paper addresses the quantification of human exposure to volatile organic compounds (VOC’s) as a result of mass transfer from tap water to indoor air. A threecompartment model is developed and used to simulate the 24-h concentration profile within the shower, bathroom, and remaining household volumes of a dwelling. Mass transfers from water to air are derived from measured data for radon and adjusted to account for the difference in mass-transfer properties for VOC’s. A preliminary data base for household parameters is used to calculate a range of concentrations and human exposures in US.dwellings. The model is used to estimate exposure factors for seven compounds-chloroform, ethylene dibromide, dibromochloropropane, methylchloroform, perchloroethylene, trichloroethylene, and carbon tetrachloride. The calculated ratio of indoor-air exposure to tap water concentration is compared to measured values for one of the compounds, chloroform. A sensitivity analysis is used to identify important parameters.
Introduction Human exposure to volatile compounds in water can occur from pathways other than direct ingestion. These pathways include inhalation of contaminants transferred to the air from showers, baths, toilets, dishwashers, washing machines, and cooking; ingestion of contaminants in food; and dermal absorption of contaminants while washing, bathing, and showering. The relative importance of these pathways has been considered as potentially important for volatile organic compounds (VOC’s) (1-5). The contribution to indoor exposures of waterborne radon-222, another highly volatile substance, also has been studied (6,7). These studies indicate that exposure to volatile chemicals from routes other than direct ingestion may be as large as or larger than exposure from ingestion alone. In this paper, I address the issue of indoor inhalation exposure attributable to VOC-contaminated tap water in the home. I begin with a review of previous research that focuses on estimates of the contribution of indoor air inhalation to overall exposure attributable to chemicals in tap water. This review is followed by a description of a three-compartment simulation model to calculate chemical concentrations within households and the resulting distribution of population exposures. The three compartments used in the model are the shower/bath stall, the bathroom, and the remaining household volume. The projected population exposure is dependent upon chemical mass-transfer rates from water to air, compartment volumes, air-exchange rates, and human occupancy factors. On the basis of data derived from U.S. housing stock information, I present a preliminary data base for the model parameters. The model is applied to seven chemicals that have been detected in California water supplies-carbon tetrachloride, chloroform, ethylene dibromide (EDB), dibromochloropropane (DBCP), l,l,l-trichloroethane (TCA or methylchloroform), tetrachloroethylene (perchloroethylene or PCE), and trichloroethylene (TCE). For one of these chemicals, chloroform, I compare model predictions of the indoor-air exposure/tap water concen1194
Envlron. Sci. Technol., Vol. 21, No. 12, 1987
tration ratio to that measured in dwellings by Wallace et al. (8, 9). A sensitivity analysis is used to identify the important parameters in the simulation model. The paper concludes with a discussion of uncertainties and variabilities as well as recommendations for future work.
Background Studies of natural radioactivity by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) (10, 11)first identified the potential significance of the water-to-air pathway for volatile substances and provided estimates of the air-to-water concentration ratio for radon in dwellings. On the basis of calculated and measured values, the 1982 UNSCEAR (11)study reports that, for indoor air contaminated by radon-rich water, the air-to-water concentration ratio (Bq/L of air per Bq/L of water) is in the range (0.4-1.5) X In the earlier UNSCEAR study (IO),this ratio was calculated to be 2.0 x 10-4. As part of their effort to estimate population exposures from radon in Houston public water supplies, Prichard and Gesell (6) compiled data on radon-to-water transfer efficiencies. The radon-to-water transfer efficiency is the becquerels per liter of radon transferred from water to air divided by the initial radon concentration in becquerels per liter. They also compiled data on household water consumption by use category and described volumes and air-exchange rates typical of Houston homes. The transfer efficiencies were determined experimentally by measurements of the radon content of water entering and leaving the house during domestic activities. Hess et al. (7) also conducted experiments to measure radon outgassing from household water use. Table I repeats and compares the radon transfer efficiencies reported in Prichard and Gesell (6)and Hess et al. (7). Also listed in Table I are estimates of water consumption rates for each use category. These values are taken from Bond et al. (15) and Siegrist (16). In a paper addressing exposure to volatile chemicals in enclosed spaces, Mackay and Paterson (12) used the “two resistance” approach to calculate the transfer of volatile species from water to air. In this approach, the rate of mass transfer from liquid to air is proportional to the overall mass-transfer coefficient K: mass transfer (mg/s) = KA(C1- C,RT/H) (1) where K = overall mass-transfer coefficient, m/s; A = contact area between liquid and gas, m2; C1 = chemical concentration in water, mg/m3; C, = chemical concentration in air, mg/m3; R = gas constant, 0.0624 Torr m3/ mo1.K T = temperature, K; and H = Henry’s law constant, Torr.m3/mol. The overall mass-transfer coefficient K at the air-water interface reflects the resistance through both the liquid and gas phases:
where Kl = liquid-side mass-transfer coefficient, m/s, and K , = gas-side mass-transfer coefficient, m/s.
0013-936X/87/0921-1194$01.50/0
0 1987 American Chemical Society
Table I. Radon Transfer Efficiency from Air to Water Reported by Prichard and Gene11 ( 6 ) and by Aess et al. (7) Together with Water Consumption by Category of Use radon transfer water efficiency, 90 consumption Prichard and Hess et al. daily use: Geaell (6) (7l Lfperson per day toilets showers baths laundry dishwasher kitchen and sinks cleaning total
30 63 47 90 90 30 90
30 65 30 90 98 30
3545 25-75' 28-44 14 19-68 13-30 134-326
'Derived from Bond and Stauh (15) and Siegrist (16). bShowers and baths reported together.
The EPA has proposed simple models for assessing respiratory exposure to contaminated water (1,13).Cothem et al. ( I ) suggest that respiratory uptake for VOC's is approximately equivalent to oral uptake (assuming 2 L ingested per 70 kg of body weight per day) with the major contribution attributable to showers or baths and the time spent in the bathmom after these activities. Assuming that a typical sbower consumes 190 L of water in a i2-m3 bathroom, Dixon et al. (13)calculated a daily inhalation exposure of 5.2 mg inhaled during a 20-min period per mg/L in water (0.074 mg/kgd for a 70-kg adult per mg/L) attributable to contaminated water. Shehata (2) evaluated multiple exposure routes to gasoline-contaminated water supplies with a two-compartment indoor-air model. The two compartments of the model household were a 450-m3house volume and ll-m3 bathroom. Shehata (2)compared the results of the model calculations to concentrations of benzene in breathing-zone air that were measured during 15-min showers in eight different households. The measured ratio of air concentration to water concentration (ppb of air/ppb of water) in the samples ranged from 0.7 to 5.6 with a mean of 2.7. On the basis of Shehata's assumptions, this corresponds to a ratio of inhalation uptake (while showering) to ingestion uptake that ranges from 0.24 to 1.9 with a mean of 0.92. Andelman (3,4) conducted experiments using a model shower enclosure to study the transfer of TCE from water to indoor air. His experiments on TCE revealed that volatilization is incomplete and that TCE concentrations in shower air increased with water temperature, drop path, and TCE concentration in water. Andelman estimates that indoor inhalation of volatile pollutants in water supplies could result in adult exposures that are 6 times higher than ingestion exposures based on an intake of 2 L/day. On the hasis of Andelman's work, Foster and Chrostowski (14) developed a model for calculating VOC concentration in shower-room air through evaluation of the mass transfer a c r m the surface of a hypothetical shower droplet. Using this model, they estimated the ratio of inhalation exposure while showering to ingestion exposure from drinking water (2 L/70 kg per day) to be in the range 1.1-2.0. A Three-Compartment, Indoor Mass Balance Model The compartment model that I developed to simulate the transfer and distribution of volatile chemicals inside homes consists of three compartments: the shower/hath stall, the bathroom, and the remaining household volume. Figure 1illustrates the major components of the model and shows the mass flow pathways that are addressed.
Flgure 1. A ihreecompa~mentmodel for slmulatlng the transfer of
contaminants from tap water to Indoor air.
Model Structure. For each compartment, I established the time-dependent chemical concentration by solving the differentialequation that balances the time rate of change of contaminant mass to the instantaneous difference hetween gains and losses. The mass balance equations for the three compartments take the following forms: for the shower stall
for the bathroom
for the remainder of the house dC.(t) V a d t
= 8 a ( t ) + qbaCb(t)
- (qab
+ qao)ca(t)
(5)
In the equations, the C.Srefer to concentrations, mg/L; the Vs refer to volumes, L; and the Q s refer to sources, mg/min. The subscripts s, b, and a are used to indicate the shower, bathroom, and remaining household compartments, respectively. The q's are used to represent air-exchange rates, L/min, with the subscripts identifying the source and end point of the transfer. For example, the mass transfer qSbrefers to an exchange from shower air, s, to bathroom air, b; qbr refers to the air exchange from bathroom air to household air. The air-exchange parameters qboand q. refer to air transfers from the bathroom and household compartments to outside air. The air-exchange parameters q are derived from the following relationships: qeb
= Qb.= Vs/Rs
Vb/Rb = Qb.+ Pbo qbo fo%b qba
Va/Ra
9.b
- qbo
= qao -k qab
(6)
(7) (8) (9) (10)
in which Rs,Rb, and R, are the residence times of air volumes in the shower, bathroom, and household air compartments and j . is the fraction of air entering the bathroom that is exhausted directly to outside air by ventilation. The parameters V., Vb, V,, R,, R,, R,, and j , are summarized in Table I1 where representative values and value ranges are provided. These values are based on the assumption of four occupants with water consumption rates Envlron.
Sci. Technol.,
Vol. 21, No. 12. 1987
1195
Table 11. Summary of Parameters Used To Calculate Indoor-Air Concentrations of VOC's Attributable to Water Use by a Family of Four
description volume of shower residence time of air in shower stall volume of bathroom residence time of air in bathroom volume of remaining house residence time of household air fraction of air leaving bathroom exhausted outdoors water used in showers and baths water used in toilets water for other household uses duration of shower per individual time when shower water use begins time when shower water use ends time when toilet water use begins time when toilet water use ends time when other household water use begins time when other house water use ends transfer efficiency from shower/bath water to air for radon transfer efficiency from toilet water to air for radon transfer efficiency from other household water use to air for radon
representative value
likely range
2000 L 1300-3000 L" 20 minb unknown 10 m3 30 rnin
6-50 m3 20-60 rnin
400 m3 150-700 ma 120 min 30-240 rnin 0.10
0-0.6
300 L
100-400 L
300 L 400 L
140-400 L 240-570 L
10 min
5-20 rnin
Calculation of Transfer Efficiencies. In the previous sections, I outlined a method for calculating chemical transfer from tap water to indoor air. Key parameters in this calculation are the transfer efficiencies &, $b, and 4,. Representative values of these transfer efficiencies based on measured values for radon by Prichard and Gesell(6) and Hess et al. (7) are listed in Table 11. In this section I use the ratio of overall mass-transfer.coefficients a t the water-air interface to derive transfer efficiencies for other volatile compounds. My goal here is not to provide a procedure for precisely defining the transfer efficiencies but instead to provide a simple method for estimating the likely range of transfer efficiencies on the basis of what we know about radon. As was shown in eq 1,the overall mass-transfer coefficient K defines the rate of mass transfer between the liquid and gas phases. As shown in eq 2, K depends on the combined mass transfer through liquid and gas boundary layers. One approach to evaluating mass-transfer coefficients in Newtonian fluids is to correlate it with the dimensionless Nusselt number, Nu (17). Using this approach, we obtain
7 a.m. 72
4T8hr 12 a.m. 12 a.m.
7 a.m. 11 p.m.
0.7
0.3-0.7
0.3
0.3
0.66
0.58-0.74
taken from Table I. The household volumes and air-exchange rates represent the range of values reported in papers described under Background. Source Terms. The source terms Q&), Q&), and Q,(t) are used to account for the input of a volatile chemical from the use of contaminated water in each respective household compartment. The general form of the source term for an arbitrary compartment i has the form
where Qi(t) = time-dependent source term in the ith compartment, mg/min; Ii= total amount of water consumed by activities in compartment i, L; q5i = efficiency for transfer (mg/L transferred divided by mg/L initial concentration) of the chemical from water to air for water use in compartment i; H(t, rp, 7;)= function whose value is 1 when t is between 7; and 7; and zero otherwise, unitless; $' = time at which activity in compartment i begins, min; = time a t which activity in compartment i ends, min; and C, = contaminant concentration in the water supply, mg/L. In the current version of the model, I combine all water uses into three activities-showers/baths in the shower stall, toilet use in the bathroom, and all other water uses Environ. Sci. Technoi., Vol. 21, No. 12, 1987
Kl = DlNul/Ll
(12)
K, = D,Nu,/L,
(13)
and
+
OFoster and Chrostowski (14). *Andelman (3).
1196
in the remainder of the house.
where D1 = the diffusion coefficient of the chemical in water, m2/s; D, = the diffusion coefficient of the chemical in air, m2/s; Nul = the water Nusselt number, dimensionless; Nu, = the air Nusselt number, dimensionless; L1 = a characteristic length in water, m; and La = a characteristic length in air, m. The Nusselt number depends on the Schmidt number Sc and on many conditions of airwater interaction: Nul
a:
Sc11J3
(14)
Nu,
a:
Sca1I3
(15)
where Scl and Sc, = the Schmidt numbers, dimensionless [Scl = pl/D1(in water) or qa/Da (in air)]; q1 = viscosity in water, m2/s; and qa = viscosity in air, m2/s. For air, qa = 1.56 X m2/s, and for water, q1 = 9.82 X lo-' m2/s a t 20 "C. Combining eq 12-15 gives
so that eq 12 can be written as
where /3 is a dimensionless constant that depends upon the physical situation but is independent of the species under consideration. The transfer efficiency for species other than radon is calculated under the assumption that the transfer efficiency is in proportion to the overall mass-transfer coefficient K a t the liquidlgas boundary. Therefore
(-2.5 +-
RT
\
where 8; = transfer efficiency for species j for water use in compartment i, unitless; and @" = transfer efficiency
Table 111. Mass-Transfer Properties for Radon a n d Seven VOC's at 20 O c a 'O''t-
compound j radon carbon tetrachloride chloroform DBCP EDB PCE TCA TCE
diffusion diffusion Henry's law coefficient coefficient constant,b in water, in air, Torr.m3/ KG)/ m2/s m2/s mol K(Rn) 1.4 x 10-9 8.2 X
2.0 x 10-6 7.8 X 10"
9.2 X 7.2 X 8.9 X 7.6 X 8.1 X 8.1 X
8.7 X 6.9 X 8.1 X 7.4 Y 7.8 X 7.8 X
10" 10" 10" 10" 10" 10"
7OC 17
1.0 0.70
2.4 0.18 743 21 3.0 8.0
0.75 0.59 0.74 0.66 0.69 0.69
3x103
t
Bathroom
air,/
1
i
\
'"'t
3X1V4
- \
-----7
Household Slr
I
I
3
6
1
I
I
9 12 15 Hour of the day
I
I
1
18
21
24
Calculated by property-estimation methods described in Lyman et al. (18)unless otherwise noted. *Estimated as the ratio of vapor pressure to solubility. Vapor pressure and solubility data are taken from Verschueren (19). "erived from Mackay and Paterson (12).
Figure 2. Estimated 24-h concentrations of PCE in household compartments resulting solely from tap water Inputs. Concentrations are calculated for the reference case.
for radon as derived from measurements, by Prichard and Gesell(6) and Hess et al. (7) for water use, in compartment i, unitless. Equation 18 allows the estimation of the relative magnitude of the water-to-air transfer efficiency of a number of compounds on the basis of measured values for radon (or any other compound). For the current estimates, I evaluate the mass-transfer parameters used in this equation a t 20 OC. Although this may seem to introduce significant error because of the sensitivity of the Henry's law constant to temperature, it should be recognized that for the compounds considered here the overall mass-transfer coefficient is dominated by liquid-side resistance, which tends to be less dependent on temperature than gas-side resistance. The model also relies on the assumption that mass transfer from shower droplets, water faucets, and toilets can be projected with a generalized correlation for turbulent mass transfer. Table I11 summarizes the parameters used to calculate the mass-transfer coefficients in eq 18 and provides the resulting ratios, K(j)/K(Rn). Daily Concentration Profiles. Equations 3-5 were solved with the PREMOD/MODAID simulation models (20). Figure 2 displays the calculated 24-h concentration profiie for PCE that was obtained with the reference values
in Table 11. For this case, the concentration of PCE in the water supply is assumed to be 1 mg/L. This figure illustrates that the concentration profile in all three compartments is driven by the source term in the shower stall, which begins at 7 a.m. After the peak concentration in the shower stall decays, the concentration in the shower, bathroom, and remaining household compartments becomes dependent on the other sources. Table IV lists the average and maximum concentrations of seven compounds in the air of three household compartments. These concentrations were calculated by using a 1 mg/L of tap water concentration, the representative values in Table 11, and the mass-transfer properties in Table 111. The average and maximum concentrations are calculated only for the period when the particular household compartment is being used. Thus, for the shower stall, the average and maximum concentrations are based on the period from 7 a.m. to 8 a.m., and for the bathroom, the maximum and average concentrations are based on the period from 7 a.m. to 9 a.m. For the remaining household volume, the maximum concentration refers to the full 24-h period. I report two average concentrations in the remaining household volume, one for daytime, from 7 a.m. to 11 p.m., and another (in parentheses) for nighttime, from 11 p.m. to 7 a.m. The results in Table IV indicate
Table IV. Average and Maximum Air Concentrations Calculated for Seven Chemicals in Each of the Three Household Compartments Using Representative Parameter Values and a Tap Water Concentration of 1 mg/L concentration, mg/L bathroom airb max av
shower air" compound
max
av
max
household airC av
carbon tetrachloride
2.7 X
1.8 X
5.1 x 10-3
3.6 x 10-3
2.3 x 10-4
chloroform
2.9
2.0 x 10-2
5.5 x 10-3
3.8 x 10-3
2.5 x 10-4
DBCP
1.0 x 10-2
9.3 x 10-2
2.2 x 10-3
1.8 x 10-3
1.2 x 10-4
EDB
2.8
X
1.9 x 10-2
5.4
x 10-3
3.8 x 10-3
2.41 x 10-4
PCE
2.5
X
1.7 X
4.8 x 10-3
3.4 x 10-3
2.2 x 10-4
TCA
2.6
X
1.8 X
5.1 x 10-3
3.5 x 10-3
2.3 x 10-4
TCE
2.6
X
1.8 X
5.1 x 10-3
3.5 x 10-3
2.3 x 10-4
X
1.2 x (2.4 x 1.2 x (2.6 X 8.0 x (2.0 x 1.2 x (2.5 x 1.1 x (2.3 x 1.1 x (2.4 x 1.1 x (2.4 x
10-4 10-5) 10-4 10-5 10-5) 10-4 10-5) 10-4 10-4 10-5) 10-4 10-5)
"For the shower, the maximum and average refer to the period from 7 a.m. to 8 a.m. when the shower is in use. bFor the bathroom, the maximum and average refer to the period from 7 a.m. to 9 a.m. when the bathroom is in use and likely to be occupied. For the remaining household volume the maximum refers to a 24-h period. The first value listed under average refers to the period from 7 a.m. to 11p.m., and the value in parentheses is the average concentration during the period 11 p.m. to 7 a.m. Envlron. Sci. Technol., Vol. 21,
No. 12, 1987
1197
that the estimated average concentration ratios for showerlbathroom, bathroom/remaining house, and shower/ remaining house are respectively of the order 10, 10, and 100.
Occupancy Factors and Human Exposure. The time-dependent concentration profiles of a chemical in shower-stall air, bathroom air, and household air are used to calculate daily human exposure and doses through inhalation. The daily dose is calculated from the formula D=
&x
24
IOF,(t)C.(t)
-
20 mi" .accvpanoy during 2 h
0.18
+ OF&)C&) +
OF,(t)C.(t)]BR(t) dt (19) where D = daily dose rate to an individual occupant of the house, mg/kgd; a = fraction of the chemical taken into the total lung volume that is available for uptake, unitless; BW = body weight of the individual, kg; OF,(t) = OCCUpancy factor for the probability that an individual is in the shower at timet, unitless; OFb(t) = occupancy factor for the probability that an individual is in the bathroom at time 1, unitless; OF&) = occupancy factor for the probability that an individual is the remaining household volume at timet, unitless; and BR(0 = breathing rate of an individual at time t, L/min. Daily dose rates are calculated for adults, children, and infanta. Adults are defined as individuals from age 16 to age 70 years, who are assumed to weigh 67 kg and breath 20 L/min for 16 h/d while active and 6.6 L/min for 8 h/d while resting. Children are defined as individuals from age 2 to age 16 years, who are assumed to weigh 32 kg and breath 13 L/min for 16 h/d while active and 4.8 L/min for 8 h/d while resting. Infanta are defined as individuals from age 0 to age 2 years, who are assumed to weigh 8.5 kg and breath 4.2 L/min for 10 h/d while active and 1.6 L/min for 14 h/d while resting. These parameters are taken from ICRP Publication 23 (21).
As for the indoor concentration profiles, two sets of assumptions are used to calculate daily dose rates. These assumptions correspond to 'reference" and 'upper-hound" assumptions. The assumptions used for reference dose estimates are as follows: Occupants spend 100% of their time in the house from 11 p.m. to 7 a.m. The bathroom is used for showers/baths from 7 a.m. to 8 a.m. Each adult and child spends 20 min in the bathroom during the period from 7 a.m. to 9 a.m. Each adult and child spends an additional 20 min in the bathroom during any 22-h period (excluding the hours 7 a.m. to 9 a.m.). Each adult spends 10 min in the shower or bath each day. Adults spend 25% of the time from 7 a.m. to 11 p.m. in the house. Children spend an average of 20 min/wk in showers or baths. Children spend 60% of the time between 7 a.m. and 11 p.m. in the house. Infanta spend 100% of their time in the house and 2% of that time in a bathroom. A total of 50% of the chemical taken into the lung is available for uptake. The assumptions used for upper-bound doses include the above set of assumptions with the following modifications: Each adult and child spends 40 min in the hathroom between 7 a.m. and 9 a.m. The bathroom is used for showers/baths from 7 a.m. to 8 3 0 a.m. llBU
.
Envlron. Scl. Technol.. VoI.
21.
No. 12, 1987
20 mi" .acovpancy during 22 h
8' 0.02
0
2
4
6
8
10
1a
12
16 18
20
'U
no",
01
tn. sa"
22
24
1
' 1
24
Figwe 3. Occupancy factors In ths shower, bafhrm, and house fa adults during a 24-h psrffl mrrespondlng to reference assumptions.
Each adult spends 20 min in the shower or bath. All age group spend all of their time in the house. Children mend an averaee of 40 min/wk in showers or baths. AU of the chemical taken into the lung is available for uptake. Figure 3 shows the 24-h profile of occupancy factors for adults in the shower, bathroom, and house corresponding to the reference assumptions. The occupancy factor for each compartment is defined as the probability that an individual is in that compartment at any specific time. For a unit concentration (C, = 1 mg/L), the doses D calculated from eq 19 can be interpreted as a unit pathway dose factor, fi, for inhalation. Thus, for an adult fi(adult) = D(adult)/C, (20) The pathway dose factors for each of the three age groups are used to calculate a lifetime-equivalent daily dose rate by taking the weighted sum:
-
The lifetime pathway dose factor Fitranslates the water supply concentration C, (in mg/L) into an equivalent daily dose rate in mg/kg.d. The factors 2/70,14/70, and 54/70 reflect the fraction of time a population cohort spends in each age category. This expression aeaumes that a population is continuously exposed to the same contaminant concentration.
Model Analysis Calculated Pathway Dose Factors. Table V lists the lifetime pathway dose factors, Fi,that were calculated with the simulation model applied to the seven VOC's under the set of reference and upper-bound assumptions described above. Because the transfer efficiency from water to air depends on the liquid-side mass transfer, which does not differ significantly among these compounds, we see that the pathway dose factors are not substantially different among six of the seven compounds. However, the pathway dose factors for DBCP are about half the others. This difference is attributable to the very low vapor pressure to solubility
Table VII. Estimates of Ratio of Indoor Inhalation Dose to Ingestion Dose for a Unit Concentration of 1 mg/L from This Simulation and Previous Work
Table V. Values of Lifetime Inhalation Pathway Dose Factors Calculated for Seven VOC's Using the Three-Compartment Model for Indoor Exposures compound
reference estimate"
upper-bound estimate"
inhalation uptake, mg/kgd/ ingestion uptake, rng1kg.d"
compd
ref
carbon tetrachloride chloroform DBCP EDB PCE TCA TCE
0.042 0.045 0.023 0.044 0.039 0.041 0.041
0.17 0.18 0.11 0.18 0.16 0.16 0.16
-1.5 to -6.0 0.8-4.0 1 2.6b 0.24-1.9' 6 1.1-2.0c
VOC'Sb DBCP VOC'S VOC'S benzene volatile pollutant TCE
this simulation this simulation 1
"Pathway dose factor for indoor inhalation attributable to a unit concentration in water supplies, (mg/kg.d)/(mg/L).
Table VI. Percentage Contribution to Lifetime Average Dose Factor Fi for Chloroform from Specific Age and Household Exposures exposure
% contribution
Reference Pathway Dose Factor adult in shower 50 in bathroom 20 in remainder of house 8.8 child in shower 5.1 in bathroom 7.0 in remainder of house 7.7 infant in bathroom 0.16 1.3 in remainder of house total 100 Upper-Bound Pathway Dose Factor adult in shower 41 in bathroom 25 in remainder of house 15 child in shower 4.2 in bathroom 8.7 in remainder of house 5.4 infant in bathroom 0.15 in remainder of house 0.62 total 100 ~
ratio of DBCP ( H / R T = 0.01). DBCP is the only compound in this set that reaches saturation concentration in the shower air. Once this happens, there can be no net mass transfer from shower water to air. Table VI summarizes the relative contribution to the lifetime pathway dose factor Fi from each age category and household compartment for chloroform. This table reveals that exposures of adults in the shower and bathroom are major contributors to lifetime inhalation exposures. Table VI1 lists the ratio of inhalation intake to ingestion intake (based on 2 L/d per 70 kg) projected from these simulations and from previous estimates. Comparison with Measured Results. In this section the results of predictions by the simulation model described here are compared with the measured TCE concentrations in a model shower enclosure by Andelman (3) and with the ratio of indoor-air concentrations to watersupply concentrations for chloroform by Wallace et al. (8, 9). Andelman measured TCE concentrations in air from a shower enclosure in which water containing 3.8 mg/L TCE flowed through the shower head for 60 min (3). During this period the TCE concentration in the shower air reached 50-80 mg/m3. On the basis of the values in Table
13 2 4 14
"Assuming 2 L/d for a 70-kg individual. bIncludes carbon tetrachloride, chloroform, EDB, PCE, TCA, and TCE. "Only includes inhalation exposure while showering.
IV, I project the TCE concentration in shower air to be in the range 68-100 mg/m3 for a water-supply concentration of 3.8 mg/L. Although this comparison suggests good agreement with experimental measurements, it should be noted that the shower enclosure used in Andelman's experiment is not necessarily representative of the range of shower configurations found in U.S. homes. Wallace et al. (8,9) measured individual exposures to about a dozen VOC's in air and drinking water. Subjects for this study were selected from New Jersey, North Carolina, North Dakota, and California. Air samples were taken indoors and outdoors with a personal air monitor. Among the compounds studied, two (chloroform and bromodichloromethane) were found to be transmitted to indoor air primarily from tap water supplies. This suggests that the air concentration measured by the personal air monitors for those compounds should be comparable to the concentrations projected by our indoor-air model. The reported mean concentration of chloroform in personal air and drinking water was 2.1 pg/m3 and 128 pg/L in New Jersey and 3.4 pg/m3 and 120 pg/L in North Carolina (8). Assuming a daily breathing rate of 20 m3 per 70 kg, these values translate to inhalation pathway dose factors of 0.005 and 0.008 (mg/kg.d)/(mg/L). In a later study where the seasonal average concentrations in New Jersey are reported for fall, summer, and winter, the personal air concentrations were 8.7,4.6, and 4.0 mg/m3 and the corresponding drinking water concentrations were 70,61, and 17 mg/L (9). These results give corresponding pathway dose factors of 0.035,0.021, and 0.067 (mg/kg.d)/(mg/L). In California studies, Wallace et al. (9) measured personal air and municipal water supply concentrations, twice in Los Angeles and once in Contra Costa County. The resulting average personal air concentrations were 1.9, 1.1, and 0.6 mg/m3 corresponding to average tap water concentrations of 14, 29, and 42 mg/L. These numbers give pathway dose factors of 0.039, 0.011, and 0.004 (mg/kg.d)/(mg/L). These measured values suggest that for chloroform the pathway dose factor should range from 0.005 to 0.07 (mg/kg-d)/(mg/L) and is low compared to my calculated values of 0.045-0.18 (mg/kg.d)/(mg/L) stated in Table V. However, it should be noted that the personal air monitors used in these measurements were not worn in the shower and probably not worn in the bathroom. The range of nonbathroom indoor exposures projected by my model are in the range 0.008-0.04 (mg/kg.d)/(mg/L) and are comparable to values derived from the measurements of Wallace et al. (8, 9). Sensitivity Analysis. A sensitivity analysis was used to identify the relative effect of each parameter on the estimate of the pathway dose factor. The local relative sensitivity factor was calculated as the percentage change Environ. Sci. Technot., Vol. 21, No. 12, 1987
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Table VIII. Sensitivity of Pathway Dose Factor for PCE to Model I n p u t s
parameter ( x ) fraction of chemical inhaled available for uptake transfer efficiency from shower water to air water use per individual in showers ratio of breathing rate to body weight (adult) duration of shower volume of shower stall amount of time spent in bathroom residence time of household air volume ratio of breathing rate to body weight (child) residence time of air in shower stall volume of bathroom residence time of bathroom air volume volume of house water consumed in other household activities transfer efficiency from household water to air household occupancy factor during the day transfer efficiency from toilet water to bathroom air water use in toilets ratio of breathing rate to body weight (infant)
local relative sensitivity, dFfdx 1.0 0.85 0.85 0.78 0.55 -0.36 0.24 0.22 0.20 0.19
-0.17 0.16 -0.14 0.094 0.094
0.078 0.055 0.055 0.020
in the overall pathway dose factor resulting from a 1% increase in an input parameter. Table VI11 lists the relative local sensitivities for the calculated pathway dose factor for PCE on the basis of a tap water concentration of 1mg/L. The results indicate that the inputs having the greatest local relative sensitivity are the fraction of inhaled chemical available for uptake, the ratio of breathing rate to body weight, the transfer efficiency from water to air, and the volume of water used in showers. Other influential parameters include shower duration, shower stall volume, amount of water used in toilets, amount of time spent in bathrooms, and residence time of the household air. Discussion a n d S u m m a r y
The study of human exposure to indoor radon identified the potential for transferring volatile compounds from household water supplies to indoor air. Recognition of the importance of water-to-air transfers in radon exposure led to efforts to determine the significance of inhalation exposure from VOC-contaminated water supplies. In this paper I have analyzed human exposure to VOC’s as a result of mass transfers from tap water to indoor air. I began with a review of the literature on this issue. Most of the previous work relied on rather simple models of this mass-transfer process. Nonetheless, these models indicated that inhalation exposures to VOC’s transferred from water to air could be as great as, or even greater than, exposures from ingestion. To provide a more comprehensive picture of the relationship between inhalation exposure and water contamination, I developed a time-dependent, three-compartment model. This model simulates the daily concentration of VOC’s in the shower, bathroom, and remaining volumes of a dwelling. Mass-transfer efficiencies from water to air were derived from measured values for radon and projected for other chemicals by using mass-transfer resistance properties. I used this model, the data on water use in U.S. homes, and a range of exposure parameters to calculate reference and upper-bound estimates of the lifetime daily equivalent human dose in mg/kg.d for a unit tap water concentration, mg/L, for the inhalation pathway. Exposure estimates are made for seven compounds-chloroform, EDB, DBCP, TCA, PCE, TCE, and carbon tetrachloride. For DBCP, the pathway dose factors corre1200
Environ. Sci. Technol., Vol. 21, No. 12, 1987
sponding to the reference and upper-bound estimates are 0.02 and 0.1 (mg/kgd)/(mg/L). For the other VOC’s, the reference and upper-bound estimates of pathway dose factors are 0.04 and 0.2 (mg/kgd)/(mg/L). These pathway dose factors suggest that indoor inhalation exposures attributable to contaminated tap water can be between 1.5 and 6.0 (0.8-4 for DBCP) times the exposure attributable to the consumption of 2 L/d tap water by a 70-kg adult. More than half of the daily inhalation exposure is projected to occur in the shower stall with an additional one-third projected to occur in the bathroom. Measurements by Wallace et al. (8,9)using personal air monitors suggest that for chloroform the nonbathroom pathway dose factor should be in the range 0.005-0.07 (mg/kgd)/(mg/L). The simulation model projects nonbathroom pathway dose factors between 0.008 and 0.04 (mg/kg.d)/ (mg/L). Although this comparison indicates good agreement between projected and measured values, it only serves to verify exposures in the nonbathroom portion of the household. We have not been able to find data to verify the projected values of shower and bathroom exposure. A local, relative sensitivity analysis was applied to the simulation model to determine the effect of parameter changes on the projected pathway dose factor. The results indicate that the pathway dose factor is most sensitive to changes in the uptake fraction in the lung, the ratio of breathing rate to body weight, the water-to-air transfer efficiency, and the quantity of water used in showers. This suggests that the full importance of the inhalation pathway may not be fully quantified until the effect of uncertainties and variabilities in these parameters can be characterized better. There are a number of areas in which further research could reduce the uncertainties and limit the variabilities in this model. Among these are three areas that offer an opportunity for the most useful research. First, there is a need to conduct a more extensive characterization of the distribution of exposures within given population groups. This would require the collection of more detailed information on the characterization of U.S. housing stock, types and numbers of shower and bathroom facilities in each home, number of baths and showers per individual per day, water flow rates, chemical uptake in the lung, water-use patterns, and occupancy factors. It would also be useful to characterize better the distribution of exposures by age of individuals exposed. Second, there is a need for a global sensitivity analysis using the simulation model with the more detailed data sets as inputs. The ranges and distributions of parameters can then be combined by use of the simulation model to produce a response surface. Relationships between the input ranges and model output should then be assessed with stepwise regression in order identify the relationship between output variability and input uncertainties and variabilities. Finally, on the basis of the results of the global sensitivity analysis, research should be directed to those parameters which, if better characterized, could most effectively reduce variability in the results. Acknowledgments
I thank David W. Layton and Kenneth T. Bogen of Lawrence Livermore National Laboratory, Lauren Zeise of the California Public Health Foundation, and Hugh Olsen of the University of California, Davis, for their consultations, comments, and reviews. Registry No. DBCP, 96-12-8; EDB, 106-93-4;PCE, 127-18-4; TCA, 71-55-6; TCE, 79-01-6; CC14, 56-23-5; CHCl,, 67-66-3.
Environ. Sci. Technol. 1987, 1 1 , 1201-1208
stances; EPA Office of Toxic Substances: Washington, DC, August 1985; Vol. 5, EPA 560/5-85-005, PB86-132156. Foster, S. A.; Chrostowski, P. C. Presented at the 79th Meeting of the Air Pollution Control Association, Minne-
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Received for review March 31,1987. Accepted July 21,1987. This paper describes work performed under the auspices of the lJ.S. Department of Energy by Lawrence Livermore National Laboratory under Contract W-7405-Eng-48. Funding was from the California Public Health Foundation and from the Harry G. Armstrong Aerospace Medical Research Laboratory, WrightPatterson Air Force Base.
Persistence of 1,2-Dibromoethane in Soils: Entrapment in Intraparticle Micropores Spencer M. Steinberg,+ Joseph J. Pignatello," and Brij L. Sawhney The Connecticut Agricultural Experiment Station, New Haven, Connecticut 06504-1 106
The soil fumigant 1,2-dibromoethane (EDB) was found in agricultural topsoils up to 19 years after its last known application. This residual EDB was highly resistant to both mobilization (desorption into air and water) and microbial degradation in contrast to freshly added EDB. Release of the residual EDB into aqueous solution was extremely slow at 25 "C but highly temperature dependent. Treatment of release as a radial diffusion process yielded effective intraparticle diffusivities of (2-8) X 1O-l' cm2/s and half-equilibration times in a 1:2 soil-water suspension of 2-3 decades at 25 OC. Aerobic degradation of residual EDB by indigenous microbes was negligible after 38 days compared to rapid removal and mineralization of added [14C]EDB. The release of residual EDB was greatly enhanced by pulverization of the soil. The results show that the residual EDB is trapped in soil micropores other than the interlayers of expandable clays where release is influenced by extreme tortuosity or steric restriction. Introduction
More than 20 million pounds of 122-dibromoethane (EDB) was used for soil fumigation in the U.S.before it was banned in 1983 by the US.Environmental Protection 'Present address: Global Geochemistry Corp., Canoga Park, CA 91303. 0013-936X/87/0921-1201$01.50/0
Agency because of its potential carcinogenicity and because it was detected in groundwater supplies. EDB is volatile (vapor pressure 13.8 mmHg and estimated Henry's law atm m3/mol at 25 O C ) (I, 2) and constant 8.2 X moderately water soluble (4250 mg/L at 25 "C) (3). It also has a low affinity for soils as evidenced by low soil-water partition coefficients Kp or carbon-referenced soil-water partition coefficients K, derived from sorption isotherms (4, this study). In addition, when added at nanogram per gram concentrations to surface soil suspensions, EDB is degraded rapidly (within days) by soil microbes under both aerobic (5)and anaerobic (6, 7) conditions. These observations suggest that EDB should disappear rapidly from surface soils following application. We have, however, found up to 200 ng/g EDB in the topsoil of tobacco fields in Connecticut, as long as 19 years after its last known application (see below). Our objective was to understand this unexpected persistence, because residual EDB could be a continued source of groundwater contamination. Our results indicate that it persists because of tenacious sorption to the soil. Neutral organic compounds are sorbed by soils and sediments by partitioning into soil organic phases and adsorption on mineral surfaces. Most experiments in the laboratory have been conducted for short times, where sorption/desorption is usually described as rapid (requiring
0 1987 American Chemical Society
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