Environ. Sci. Techno/. 1993,27,388-397
Sonoma Technology, Inc., Santa Rosa, CA, 1990. Stockburger, L.; Knapp, K. T.; Ellestad, T. G. Overview and analysis of hydrocarbon samples during the summer Southern California Air Quality Study. Presented at the 82nd annual meeting of the Air & Waste Management Association, Anaheim, CA, 1989; Paper 89-139.1. Ozone modeling-performance evaluation. Draft Technical Report V-B, Air Quality Management Plan, 1991 Revision. South Coast Air Quality Management District, El Monte, CA, 1990. Winer, A. M.; Peters, J. W.; Smith, J. P.; Pitts, J. N. Environ. Sci. Technol. 1974, 8, 1118-1121. Lonneman, W. A.; Seila, R. L.; Ellenson, W. Speciated hydrocarbon and NO, comparisons a t SCAQS source and receptor sites. Presented at the 82nd annual meeting of
the Air & Waste Management Association, Anaheim, CA, 1989; Paper 89-152.5. (51) Tesche, T. W.; Georgopoulos, P.; Seinfeld, J. H.; Cam, G. R.; Lurmann, F. W.; Roth, P. M. Improvement of procedures for evaluating photochemical models. Report to the California Air Resources Board under Contract A832-103. Radian Corp., Sacramento, CA, 1990. (52) DaMassa, J. Technical guidance document: photochemical modeling. Technical Support Division, California Air Resources Board, Sacramento, CA, 1992.
Received for review June I , 1992. Revised manuscript received October 15, 1992. Accepted October 26, 1992. This study was sponsored by the Coordinating Research Council (CRC)under Project SCAQS-8.
Monitoring of Malathion and Its Impurities and Environmental Transformation Products on Surfaces and in Air Following an Aerial Application Mark A. Brown,' Myrto X. Petreas, Howard S. Okamoto, Thomas M. Mlschke, and Robert D. Stephens
California Department of Health Services, Hazardous Materials Laboratory, Berkeley, California 94704 Concentrations in air and on surfaces of malathion and impurities [malaoxon, isomalathion, O,O,S-trimethyl phosphorodithioate, O,O,O-trimethyl phosphorothioate, O,O,S-trimethyl phosphorothioate, and diethyl fumarate (DEF)] were determined at three sites during and for 9 days after a 1990 aerial spraying in California. Malathion spray was transformed via oxidation to malaoxon, hydrolysis to DEF, and selective volatilization. Malathion half-lives were 1.6 to >9 days, and depositions at three sites ranged from 1100 to 2413 pg/ft2, while malaoxon ranged from 2.9 to 6.0 pg/ft2. Malaoxon increased fastest on filter paper surfaces, e.g., initial deposition of 7 pg/ft2 increasing to 315 pg/ft2 after 9 days. Malathion air concentrations increased to 80 ng m3 within 1day, and malaoxon to 64 ng/m3 at 24-48 h 0th were detectable after 9 days. DEF surface levels increased over 9 days; air concentrations persisted for 72 h. The three phosphoro(di)thioate trimethyl esters were undetectable on surfaces within 24 h, producing an air burst apparently via rapid volatilization. Accurate human exposure assessment during malathion spraying must consider environmental transformations. W
L
Introduction In 1989 and 1990 the California Department of Food and Agriculture (CDFA) conducted aerial applications of a malathion/protein bait mixture in urban areas of southern California in an effort to eradicate an outbreak of the Mediterranean fruit fly. Human exposure to malathion and malathion-related impurities was estimated by assuming that relative concentrations of these compounds measured in the spray mixture did not change after aerial application. During these applications, CDFA monitored the deposition of malathion and malaoxon on a single type of surface and concentrations in air for a maximum of 48 h. The California Department of Health Services (CDHS) initiated a parallel study during a single spray episode in May 1990 to monitor malathion and six impurities with known mammalian toxicities, on three different surface types and in air over a 10-day time period. The study was designed to estimate amounts and variations in the con~~~
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*Current address: Congress of the United States, Office of Technology Assessment, 600 Pennsylvania Ave. S.E., Washington, DC, 20003. 388 Environ. Scl. Technol., Vol. 27, No. 2, 1993
centrations of malathion and its impurities in the technical grade malathion concentrate and in the concentratelbait formulated tank mixture and the environmental fate of malathion and related impurities present initially in the spray mixture, and after deposition on three types of surfaces, and in air for a period of 9 days following aerial application. The malathion-related impurities measured in this study were selected on the basis of information about their relative acute and chronic toxicities. Unfortunately, most available data come from animal studies and far less information is available about human toxicities for these compounds. Acute Toxicity of Malathion and Related Impurities. Malathion, introduced in 1950 by American Cyanamid Co., is a nonsystemic organophosphorus insecticide (1). Its low acute mammalian toxicity is due to its selective hydrolytic degradation, via mammalian carboxylesterases, versus its oxidative activation to the potent acetylcholinesterase inhibitor malaoxon in insects. Manufacture of malathion results in production of various impurities (Table I). The structures and abbreviations of some of the most toxicologically relevant malathion impurities are shown in Figure 1. The increased and highly variable rat oral toxicity of technical versus highly purified malathion correlates specifically to concentrations of the impurity isomalathion (2). A direct correlation was shown between the inhibition of mouse serum or liver malathion carboxylesterase activity and LD5,,'s to rats and mice treated with technical malathion (3). The malathion impurities isomalathion, OSS(O), OOS(O), and OOS(S) increase malathion acute rat oral toxicity and act as effective inhibitors of rat liver carboxylesterases that are involved in the hydrolytic detoxification of malathion ( 4 ) . Small amounts of these impurities significantly increase the rat acute oral toxicity of technical malathion; e.g., 1% isomalathion or OSS(0) increases malathion toxicity 6- or 12-fold, respectively (2). Isomalathion and related 0,O-dimethyl phosphoro(di)thioate ester impurities are significantly more potent inhibitors of mammalian acetylcholine esterases than malathion, with bimolecular inhibition constants (&) approximately 1000 times greater than malathion (5). In contrast to the rat, only isomalathion is effective in inhibiting human liver carboxylesterase at concentrations
0013-936X/93/0927-0388$04.00/0
0 1993 American Chemlcal Society
Table I. Impurities in the Technical Grade Malathion Concentrate,O Percent Concentration of Each, and Rat Oral Toxicities (LD,,'s) As Compared to LD50for Malathion compd
LDm (mg/kdb
re1 to malathion
>90 0.20 0.10
10700 113 158
1.ox 95x 68X
0.90 0.003d 0.04d 1.2
?'
?
26 60 638
412X 178X 17X
0.45 0.05 0.03 0.15 1.0
? ? ? ? ?
? ? ? ? ?
concn (% )
malathion isomalathion malaoxon diethyl fumarate O,S,S-trimethyl phosphorodithioate [OSS(O)] O,O,S-trimethyl phosphorothioate [OOS(O)] O,O,S-trimethyl phosphorodithioate [OOS(S)] O,O,O-trimethyl phosphorothioate [OOO(S)] diethyl hydroxysuccinate ethyl nitrite diethyl mercaptosuccinate diethyl methylthiosuccinate
Data supplied by a manufacturer. bData from Aldridge et al. (2) except for malaoxon (3). C h i t a n t . dData from Fukuto (6). Table 11. Literature Reported Hydrolysis Rates for Malathion under Aqueous Conditions at Various pH's and Temperatures
CHO
CH30-P-
II
I
CHP
OCHg
1
O.O.O-TrlmethvlO.O.S.Trimethvl
O.S.S-Trlmethvl
IpppLsu
LczsX!a
ehpaphorodlthloa&
0
CWS
3
C HS
LsLQXsJ
I
I
0
PhosDhorodlihloate
[pQs(p11
-37
o v
0
0
FhQamm& DEI LQQQu Flgure 1. Structures of malathion and some of As Impurities and envlronmental transformation products with abbreviations (if used) in brackets.
less than lo+ M (6). Isomalathion may be the most significant impurity relative to the acute human toxicity of malathion (7). Isomalathion contained in technical malathion was the major determinant in an accidental poisoning of 2800 (including 5 deaths) of 7500 applicators in Pakistan in 1976 (2). Chronic Toxicity of Malathion and Related Impurities. Malathion is not considered an animal carcinogen although less is known about some of its impurities (8). Malathion and four impurities, isomalathion,OOO(S), OOS(O),and OSS(O), are not mutagenic in Salmonella typhimurium with or without activation by S9 liver homogenate, although they are chemical alkylating agents (9). One impurity, OOS(O), covalently binds to lung, liver, kidneys, and ileum tissues with a concomitant depletion of glutathione in rats (10). Long-term exposure to OSS(0) at low doses enhances the ability of mice to generate an immune response, while higher doses may suppress the generation of an immune response (11). Delayed toxicity observed with OOS(0) and OSS(0) malathion impurities showed symptoms including weight loss and death occurring up to 3 weeks after dosing. The liver was consid-
half-life ( t l l z )
conditions
ref
240 h 4 years 7.8 h 20 h 480 h 62 h 36 h 12 h
pH 2.6 distilled water pH 4.0 distilled water (est) pH 6.0 ethanol/buffer 1:4, 70 "C normal river water pH 7 (est) pH 7.5 seawater pH 8.0 seawater pH 8.0 distilled water pH 9.0 aqueous
14 14 34 14 35 35 14 36
ered a likely target organ based on a dose-dependent 1531-fold increase of liver endoplasmic reticulum derived @-glucuronidasein serum after treatment with 60 and 40 mg/kg oral doses of OOS(0) and OSS(O),respectively (12). Malathion Hydrolysis. Although malathion can undergo chemical degradation through hydrolysis in soil and water as influenced by pH, temperature, soil minerals, organic matter, and moisture, the magnitude of microbial degradation is probably far greater (13). Malathion is more stable in natural fresh and saline waters between pH 5 and 7 and less stable at pH's greater than 7 or less than 2 (Table 11). Environmental degradation of malathion via acid hydrolysis may not be significant. Thus, the half-life of malathion at pH 4 was reported to be too large to measure conveniently but was estimated to be greater than 4 years (14).Malathion formulation with a pH 4.5 protein bait, as used in the 1989-1990 CDFA spraying, may act to retard normal hydrolytic degradation by alkaline materials present in soils. Alkaline hydrolysis is much faster (Table 11) and probably is a significant environmental degradation route. Alkaline hydrolysis occurs at the ethyl ester linkages, producing the malathion mono- and dicarboxylates, and via a reverse Michael reaction, producing 0,O-dimethyl phosphorodithioate and diethyl fumarate. Malathion hydrolysis rate increases 4-fold with each 10 O C temperature increase (13). Salt concentration does not affect aqueous hydrolysis half-lives (14). Isomalathion is similarly unstable under aqueous alkaline conditions, with a half-life of 3130 h at pH 6.0 and 97 h at pH 8.0 (at 25 "C)(15). Similarly, temperature and pH are more important for alkaline hydrolysis rates than salt concentration, although increasing the salt concentration from 0.01 to 0.1 M decreases the half-life by 50% (15).
Photochemical Transformations of Malathion. Pure malathion on photolysis was less toxic than the original material although products were not identified (16). Exposure of malathion, as a thin film in a glass petri dish Envlron. Sci. Technol., Vol. 27, No. 2, 1993
389
covered with Teflon (to reduce volatilization), to sunlight or ultraviolet light yielded related toxic trimethyl phosphorothioate and phosphate esters OOS(0) and OOO(0). Photolysis rates under these conditions were maximally 0.0008-0.1% after sunlight irradiation for 25 h (17). It is difficult to extrapolate these rates to those occurring under actual environmental conditions. Pure malathion, lacking a good chromophore, absorbs very little at sunlight wavelengths. Photolysis rates may be substantially faster under more relevant gas-phase reaction conditions and in the presence of photochemical sensitizers,oxidants, water, and particulates. Thus, the photolysis half-life for malathion was 990 h in distilled water (pH 6, >290 nm) but was substantially accelerated to 16 h in natural waters containing colored, light-absorbing materials (14). Malathion is somewhat volatile, and transformations may be expected in the vapor phase. More information is required to understand the environmental photochemistry of malathion. Malathion has a higher vapor pressure compared to parathion (1.25 X lo4 versus 3.78 X 10" Torr at 20 "C). Volatilization of technical parathion applied to a surface at 298 Ng/cm2 was 0.21 pg cm-2 h-l at 25 "C. At this rate 10 pg/cm2 would volatilize in 2 days. For each 10 "C increase in temperature there was a 3-4-fold increase in vapor pressure (18). Water evaporation from soil surfaces also affects volatilization of parathion and paraoxon, with a more rapid volatilization from wet versus dry soils (19). Environmental Oxidation of Malathion. Oxidation of malathion to malaoxon in air was rapid during CDFA aerial applications (20). Two days after spraying at one measurement site the concentration of malaoxon in outdoor air was greater than that of malathion. Malathion air concentrations increased during the 2 days following aerial application, presumably by volatilization from the bait droplets on various surfaces (21). Malathion may be oxidized to malaoxon in the atmosphere via ozone or other possible oxidants such as oxides of nitrogen. Malathion and other P=S-containing organophosphorus insecticides are oxidized to their P=O oxon analogs by various chemical oxidants including bromine water, dinitrogen tetraoxide, N-bromosuccinamide, nitric acid, peracetic acid, hydrogen peroxide, and potassium permanganate but not directly by molecular oxygen (14,22,23). The malathion analog, parathion, that contains the P=S moiety has been shown to be oxidized to the corresponding and more toxic P=O malaloxon analog under a wide variety of conditions, including via ozone (24). The parathion vapor half-life in air with ozone and sunlight is less than 2 min (25). Parathion P=S oxidation on airborne particulates may be a contributing factor in field worker toxicity cases (13). On soil, P=S oxidation is highly correlated to atmospheric ozone concentrations (
