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Decreased Human Respiratory Absorption Factors of Aromatic Hydrocarbons at Lower Exposure Levels: The Dual Effect in Reducing Ambient Air Toxics Zhonghui Huang, Yanli Zhang, Qiong Yan, Zhaoyi Wang, Zhou Zhang, and Xinming Wang Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.7b00443 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017

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Environmental Science & Technology Letters

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Decreased Human Respiratory Absorption Factors of Aromatic Hydrocarbons

2

at Lower Exposure Levels: The Dual Effect in Reducing Ambient Air Toxics

3 4 5 6 7 8 9 10 11 12

†,‡

Zhong-Hui Huang, Yan-Li Zhang, † ,†,§ Zhang, and Xin-Ming Wang*

†,§

Qiong Yan,

‖

Zhao-Yi Wang,

†,‡

Zhou

†

State Key Laboratory of Organic Geochemistry and Guangdong Key Laboratory of Environmental Protection and Resources Utilization, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China ‡

University of Chinese Academy of Sciences, Beijing 100049, China

§

Center for Excellence in Urban Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China ǁ

Department of Respiratory Diseases, Guangzhou No.12 People’s Hospital, Guangzhou 510620, China

13 14 15 16 17 18 19 20 21

*Corresponding author:

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Dr. Xinming Wang

23

State Key Laboratory of Organic Geochemistry

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Guangzhou Institute of Geochemistry, Chinese Academy of Sciences

25

Guangzhou 510640, China

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Tel.: +86-20-85290180; fax: +86-20-85290706.

27

E-mail: [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT

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Respiratory absorption factors (AFs) are important parameters for assessing human health

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risks of long-term inhalation exposure to low-level hazardous air pollutants. However, it is

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uncertain whether previously measured respiratory AFs for high-level exposures could be

32

directly applied. Here we measured real-time respiratory AFs using proton transfer reaction

33

time-of-flight mass spectrometry (PTR-TOF-MS) for 50 fifty subjects (aged 20-30; 24

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females and 26 males) exposed in a normal office room with aromatic hydrocarbons (AHs) at

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concentration levels of several part per billion by volume (ppbv). The mean respiratory AFs

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of benzene, toluene, and C8-aromatics (ethylbenzene and xylenes) from all subjects were

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28.2%, 63.3%, and 66.6%, respectively. No gender difference in the respiratory AFs of AHs

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was observed. Correlation analysis revealed that exposure concentration, rather than

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physiological parameters like body mass index (BMI) or body fat ratio (BFR), was the

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dominant factor influencing the AFs of AHs. The results also demonstrated that respiratory

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AFs decreased in a logarithmic way when lowering exposure levels of AHs. The decreased

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respiratory AFs at lowered exposure levels suggest the dual effect of reducing ambient air

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toxics like AHs on lowing human inhalation intake.

44 45

Keywords: Inhalation exposure; respiratory absorption factors; volatile organic compounds

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(VOCs); hazardous air pollutants; PTR-TOF-MS

47 48 49 50 51 52 53 54 2


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There has been an increasing attention about volatile organic compounds (VOCs) in ambient

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air, not only for their roles in forming tropospheric ozone and secondary organic aerosols

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(SOA),1-4 but also for their potential carcinogenic and non-carcinogenic health effects on the

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human population. Exposure to hazardous VOCs might be associated with respiratory,

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cardiovascular, and neurological diseases including asthma, chronic obstructive pulmonary

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disease (COPD), and leukemia.5-12 The inhalation intake of toxic VOCs, however, depends

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not merely on the ambient levels of VOCs. Respiratory absorption factors (AFs), which

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denote the percentages of inhaled toxicants retained authentically inside the human body, are

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considered to be indispensable parameters in assessing daily intakes and health risks due to

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exposure to toxic VOCs.13-18

INTRODUCTION

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The air we breathe contains a diverse range of low-level VOCs that can be taken up by the

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body, and health endpoints related to long-term low-level exposure of air toxics is a

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challenging issue in environmental health. Yet very little is known about the respiratory AFs

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of low-level VOCs in indoor or outdoor environments. Results from the Total Exposure

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Assessment Methodology (TEAM) studies conducted in the 1980’s indicated that higher

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fractions of inhaled benzene concentrations are absorbed at very low doses.19, 20 Based on a

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wide range of earlier studies concerning respiratory AFs of VOCs in very high levels (tens of

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ppmv or even higher),21 AFs were assumed to be 90%,14-17, 22-25 100%18, 26-38 or most recently

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to be 50-60%39, 40 when assessing health risks of inhalation exposure to toxic VOCs. It is

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questionable whether these AFs can be applied to low-level exposure situations. Moreover,

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AFs are regarded as constant to simplify the inputs in pharmacokinetic models studying the

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fate of exogenous VOCs within the human body.41-44 In essence, these AFs are more likely to

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be variable as they are affected by a multitude of factors including exposure concentrations,

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physicochemical behaviors of VOCs and individual human physiological conditions.45 Hence

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it is necessary to determine respiratory AFs of toxic VOCs particularly in the low-level

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exposure environments. 3



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The most common aromatic hydrocarbons (AHs), namely benzene, toluene, ethylbenzene,

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and xylenes (BTEX), were chosen as typical toxic VOCs in the present study. BTEX are a

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major class of hazardous air pollutants: benzene is a well-known carcinogen causing leukemia,

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and TEX may deteriorate developmental, nervous, and heart and blood vessel systems.13, 46

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Additionally, BTEX are ubiquitous in both indoor and outdoor environments, particularly in

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developing countries.47-53 Even at background sites in China ambient benzene levels might

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exceed the limit set by European Union (EU).54 In this study, fifty volunteers were asked to

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stay in a normal office room, and by using a homemade online breath sampling device

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coupled to a proton transfer reaction time-of-flight mass spectrometry (PTR-TOF-MS),

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real-time respiratory AFs were measured for all subjects exposed to indoor AHs at several

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part per billion by volume (ppbv). The purposes of this study are: 1) to check if there is a

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gender difference in the respiratory AFs of AHs; 2) to explore the relationship between AFs

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and exposure levels under low-level exposure situations; and 3) to investigate if physiological

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factors, such as body mass index (BMI) and body fat rate (BFR), influence the respiratory

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AFs.

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MATERIALS AND METHODS

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Subjects. First phase test: a total of fifty young volunteers, who were then all graduate

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students studying in the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences,

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participated in this study. Every test subject was required to stay in a well-ventilated normal

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office room for about half an hour during the test with PTR-TOF-MS. It should be noted that

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this was not a human toxicity test because none of the BTEX were injected into the office

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room. All subjects gave written informed consent prior to participation in the study. Subjects

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completed a brief questionnaire concerning needed information regarding their gender, age,

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height, weight, BMI, BFR, smoking/drinking status, and personal/familial past medical

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history. These fifty subjects included 26 males and 24 females, aged 20-30 years old. They

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were all non-smokers and non-drinkers. Demographic data of the subjects represented in the

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study are summarized in Table S1 (Supporting Information).

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Second phase test: in order to further verify the relationships between exposure 4


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concentrations and AFs of BTEX, one male and one female subjects were randomly selected

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from volunteers participating the first phase test. They were asked to stay 4 h per day in the

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same office room over 3 days for more tests of AFs with the variation of BTEX largely due to

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ventilation.

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Online Breath Sampling and Measurement. A homemade online breath sampling

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device was used for sampling air inhaled and exhaled by subjects. Detailed description of the

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sampling device can be found in our previous study.21 There is, however, an improvement

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that a Swagelok plug valve was added between the left end of the tube and the nose interface

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in the current study.

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A commercial high-sensitivity PTR-TOF-MS (model 2000; Ionicon Analytik GmbH,

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Innsbruck, Austria) was deployed to measure BTEX levels in the breath samples. The

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measurement principle of PTR-TOF-MS has been described elsewhere in detail.55-57 The

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PTR-TOF-MS acquired data at a 0.5 Hz time resolution with H3O+ reagent ion. The drift tube

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of the instrument was operated at a voltage of 610 V, pressure of 2.20 mbar, and temperature

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of 60°C, with an E/N ratio of about 139 Townsend (Td) (where E is the electric field strength

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and N is the number density of a neutral gas; 1 Td = 10−17 V cm2).

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Detailed online breath sampling and measurement steps were described in our

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previous study.21 Briefly, indoor air, which was the air inhaled by subjects, was firstly

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sampled and measured through the online breath sampling device. During the ensuing exhaled

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air measurements, the subject steadily exhaled alveolar air into the sampling device, and then

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closed the plug valve right after a complete expiration. The plug valve was blocked until the

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exhaled air in the buffer tube was exhausted and hereafter the indoor air was extracted and

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measured for several minutes. According to the above operating steps, the inhaled and

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exhaled air for each subject was continuously measured to determine their respiratory AFs as:

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=

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where Ci and Ce (ppbv) were the concentrations of the target compound in the inhaled and

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exhaled air, respectively. Isoprene was used as a breath tracer for identifying the expiratory

× 100%

(1)

5



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and inspiratory phases in this study.21

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Quality Assurance and Quality Control. Target VOCs were identified based on their

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exact mass to charge ratio (m/z) and quantified by external calibration methods. Mass

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calibration was performed using two ion peaks with known exact masses: hydronium ion

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isotope (H318O+; m/z 21.022) and protonated 1, 2, 4-trichlorobenzene ((C6H3Cl3)H+, m/z

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180.937). The isomeric ethylbenzene and xylenes, all with a molecular mass of 106 amu,

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cannot be distinguished by PTR-TOF-MS, and thus these C8-aromatics were reported as their

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sum.

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Background levels for target compounds were determined by introducing zero air into the

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instrument. Multi-point calibration of the PTR-TOF-MS was carried out before the breath air

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measurement using VOC standard mixtures (including isoprene, benzene, toluene, o-xylene)

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that were dynamically diluted to five levels (2, 5, 10, 15, and 20 ppbv) from a certified

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standard gas mixture (Ionicon Analytik GmbH; ~1 ppmv). The linear correlation coefficients

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(R2) of calibration curves were 0.996–0.999 for BTEX compounds. Their sensitivities,

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indicated by the ratio of normalized counts per second (ncps) to the levels of BTEX in ppbv,

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were 27, 35, and 40 ncps/ppbv for benzene, toluene and C8-aromatics, respectively. The

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method detection limits (MDL) for benzene, toluene, and C8-aromatics in 2 s integration time

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were 0.055, 0.044, and 0.039 ppbv, respectively. The measurement precisions and accuracies

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were determined by repeated analysis of a standard mixture (1 ppbv) seven times. The relative

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standard derivations for BTEX were all < 5%, and the accuracies of BTEX were all within ±

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10%. BTEX will accept a proton from H3O+, but their reaction with (H2O)2H+ is

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thermodynamically unavailable. Previous studies have shown no significant humidity

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dependence on their sensitivities.58-61 To re-confirm this, three levels of standard mixtures in

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the range of 0-10 ppbv were prepared at relative humidity (RH) of 20% and 95%, respectively.

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RH was controlled as per the details provided in Kumar and Sinha.62 As shown in Figure S1,

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no significant differences in the BTX sensitivities (ncps/ppbv) were observed between the

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standard mixtures at RH of 20% and 95% probably due to a high proportion of H3O+ ion in

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the drift tube with a high E/N ratio. Thus humidity effects in breath samples can be ignored 6


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when measuring BTEX by PTR-TOF-MS in the present study.

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Statistical analysis. The Student’s t-test in the statistical software package SPSS (Version

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19) was used to examine statistical differences in measured concentration levels. Two-tailed

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tests of significance were used, and p < 0.05 indicated statistical significance. It was also used

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as critical value for significant correlation.

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RESULTS AND DISCUSSION

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Real-time AFs of BTEX. The measured BTEX exposure levels ranged from 0.19–3.26

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ppbv for benzene, 0.35–8.72 ppbv for toluene, and 0.31–6.84 ppbv for C8-aromatics in the

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first phase test. Five sets of inhaled and exhaled air for each subject were successively

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measured to acquire the respiratory AFs of BTEX. The average respiratory AFs and their

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standard deviations (SD) of benzene, toluene, and C8-aromatics from all 50 subjects were

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28.2% (SD = 10.9%), 63.3% (SD = 12.7%), and 66.6% (SD = 10.6%), respectively (Figure

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S2). The mean respiratory AFs of benzene was much lower than previously assumed or

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measured 90%,14-17, 22-25 100%18, 26-38 or 50-60%21, 39, 40; for toluene and C8-aromatics, the

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mean values were near that measured in our previous study,21 but still much lower than

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previously assumed values of 90%14-17, 22-25 or 100%18, 26-38 when assessing inhalation health

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risks.

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The mean respiratory AFs of benzene, toluene and C8-aromatics were 28.0%, 57.6% and

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63.4% for the female subjects; and 28.5%, 69.1% and 69.7% for the male subjects,

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respectively. No significant difference (p > 0.05) was observed in the respiratory AFs between

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the female and male subjects, implying no gender difference in the respiratory AFs. In our

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previous preliminary tests with 7 subjects,21 the 3 female subjects had significantly higher

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respiratory AFs of BTEX than the 4 male ones (p < 0.05). Probably the small numbers of

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subjects for the test in our previous study statistically bias the gender difference discussion.

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Influencing Factors of AFs. Three aspects of factors, i.e., physicochemical properties of

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VOCs, individual human physiology, and environmental factors, govern the respiratory

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AFs.45 In this study we only focused on BTEX and their mean respiratory AFs appeared to

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increase with molecular weight, reflecting the influence of physicochemical properties of 7



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these compounds such as lipophilicity, solubility in blood, and blood/air partition

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coefficients.41, 63, 64

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Humans are incredibly diverse, and common individual physiological parameters such as

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gender, age, health state, BMI, and BFR needed to be seriously considered for discerning the

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crucial influencing factors of AFs.41, 65, 66 As mentioned above, gender was not a key factor

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affecting the AFs. Regarding age and health state, all volunteers were healthy young people

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aged 20-30 without any past personal or familial medical history (Table S1). BMI, defined as

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a person’s weight in kilograms divided by the square of one’s height in meters (kg/m2), is a

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universal standard introduced by the World Health Organization for assessing the body fat

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levels and health state. BFR is another parameter of reflecting the percentage of body fat

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content to the body weight (Table S1). Figure 1 shows scatter plots of the respiratory AFs

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versus BMI and BFR. No significant correlations between AFs and BMI/BFR were observed,

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either for all subjects or for female and male subjects individually.

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Environmental factors including pre-exposure concentration, exposure concentration and

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duration, also influence the absorbed dose of toxic VOCs.65, 67 Because all subjects were then

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graduate students working and living in the same institute campus, they should have quite

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similar pre-exposure experience. During our test, subjects were exposed to different levels of

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BTEX in the normal office room due diurnal variations. The correlations between the AFs

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and exposed BTEX concentrations for all volunteers are shown in Figure 2a-c. The highly

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significant log-based (p < 0.001) correlations between the AFs and exposure levels of BTEX

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suggested that exposure levels rather than individual physiological factors were responsible

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for the AFs, consistent with the conclusions in some previous studies.65, 68-70

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Relationship between Exposure Levels and AF. The second phase test with just one male

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subject and one female subject for extensive measurements would eliminate the effects of

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inter-individual physiological variations. The relationships between exposure levels and AFs

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for the two subjects are illustrated in Figure 2d-i, confirming the highly significant

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logarithmic correlations for all subjects as discussed above.

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The mechanism for the logarithmic relationship remains unexplained so far. But the 8


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phenomenon is reasonable because at low-exposure levels the inhalation absorption process

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of BTEX might be similar to the Langmuir isothermal adsorption process of VOCs on the

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surface of adsorbents, in which the adsorption efficiencies would increase with the elevated

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concentrations of VOCs until reaching a relatively stable value when concentrations of VOCs

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exceed a certain level.71 As showed in Figure S3, the relationships between exposure levels

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and AFs could be well fitted with Langmuir adsorption isotherms.

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Our results, as showed in Figure 2, demonstrated that if BTEX concentrations go down to

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about 2 ppb or even lower, respiratory AFs decrease rapidly, implying the dual effect in

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lowering human inhalation dose by reducing BTEX concentration in ambient air: inhalation

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uptake would further reduced by lower AFs at lower exposure levels. This is very important

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for some air pollutants, such as benzene, that are carcinogenic to humans and no safe level of

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exposure can be recommended, although the inhalation minimal risk level (MRL) of 3 ppbv

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was recommended by the United States Environmental Protection Agency (USEPA) for

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benzene,72 and an annual limit of 5 µg/m3 (or 1.6 ppbv at 25°C and 1 atm) was established by

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the EU for benzene in ambient air.73 From our study, as showed in Figure 2a, if benzene

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levels decrease from 1.0 ppbv to 0.5 ppbv, its AFs would decrease from ~70% to ~40%,

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consequently the internal intakes would decrease by ~70%, more than the 50% expected due

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to a decrease in exposure levels alone.

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We observed that the AFs decreased in a logarithmic way with decreasing the exposure

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levels of BTEX. The finding is valuable for rationally assessing human health risks of

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long-term inhalation exposure and for evaluating the effects of control measures for BTEX.

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Nonetheless, since in this study all subjects shared similar demographic characteristics (age,

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weight, height, and etc.) and lived in the same area, it is of concern whether our conclusions

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can be applied to the general population and this needs to be verified with more extensive

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study in the future.

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ASSOCIATED CONTENT

9



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Supporting Information

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Demographic data of subjects in this study (Table S1); Normalized sensitivities at

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different relative humidity of 20% and 95% for benzene, toluene and o-xylene. Slopes

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(i.e. sensitivities) are indicated by mean values ± standard errors (Figure S1);

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Respiratory AFs of benzene, toluene, and C8-aromatics from fifty test subjects. The

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columns and their error bars represent the mean values and standard deviations of the

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respiratory AFs of BTEX, respectively (Figure S2); Extended Langmuir isotherms

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between respiratory AFs and exposure concentrations of benzene (open squares),

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toluene (open cycles), and C8-aromatics (open triangles) collected from all (blue, a-c), a

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male (cyan, d-f) and a female (green, g-i) subjects, respectively. Curve-fitting equations

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and their correlation coefficient (R2) and significance levels (p) were also presented

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(Figure S3).

260 261

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Corresponding Author

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*Phone: +86-20-85290180. Fax: +86-20-85290706. E-mail: [email protected].

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ORCID

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Zhong-Hui Huang: 0000-0003-0144-0852

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Yan-Li Zhang: 0000-0003-0614-2096

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Xin-Ming Wang: 0000-0002-1982-0928

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Notes

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The authors declare no competing financial interest.

AUTHOR INFORMATION

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This work was financially supported by the Natural Science Foundation of Guangdong (Grant

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No. 2016A030313164), the Health and Family Planning Commission of Guangzhou

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Municipality (Grant No. 20161A010050), and the Natural Science Foundation of China

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(Grant No. 41530641/41571130031). The authors would like to express their sincere thanks

ACKNOWLEDGMENTS

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to volunteers in Guangzhou Institute of Geochemistry, Chinese Academy of Sciences for their

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supports.

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19



505 506

Figure 1. Scatter plots of respiratory AFs of benzene, toluene, and C8-aromatics versus BMI

507

and BFR, respectively.

508 509 510 20


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511 512

Figure 2. Regression analysis between respiratory AFs and exposure concentrations of

513

benzene (open squares), toluene (open cycles), and C8-aromatics (open triangles) collected

514

from all (blue, a-c), a male (cyan, d-f) and a female (green, g-i) subjects, respectively.

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Curve-fitting equations and their correlation coefficient (R2) and significance levels (p) were

516

also presented. R2 > 0.5 and p < 0.05 were used as critical values for significant correlations.

517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 21



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For Table of Content Use Only

534

Decreased Human Respiratory Absorption Factors of Aromatic Hydrocarbons

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at Lower Exposure Levels: The Dual Effect in Reducing Ambient Air Toxics

536 537 538

†,‡

Zhong-Hui Huang, Yan-Li Zhang, † ,†,§ Zhang, and Xin-Ming Wang*

†,§

Qiong Yan,

‖

539 540

22


Zhao-Yi Wang,

†,‡

Zhou

Decreased Human Respiratory Absorption Factors ... - ACS Publications

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