Occupational Exposure to Formaldehyde and Genetic Damage in the Peripheral Blood Lymphocytes of Plywood Workers

Abstract

Objectives: We sought to clarify the association of occupational formaldehyde exposure with DNA strand breaks, chromosome damage and DNA-protein crosslinks (DPCs) in the peripheral blood (PB) lymphocytes of plywood workers. Methods: We determined Olive tail moment (OTM) values, micronucleus (MN) frequencies and DPC rates of the PB lymphocytes in 178 workers divided into control and lower and higher exposure groups according to their current formaldehyde exposure levels and examined the association of each end point with formaldehyde exposure levels and with the number of work years. We also examined each end point in an additional 62 workers before and after an 8-hour formaldehyde exposure for validating the association. Results: OTM values increased significantly in the two exposure groups compared with those in the control group (p<0.05 for both) and were associated with increasing formaldehyde exposure levels (p_trend=0.002), while MN frequencies increased with increasing numbers of work years (p_trend<0.001). The dynamic study showed that OTM values and DPC rates increased after an 8-hour formaldehyde exposure compared with those before the exposure (p<0.001, p=0.019, respectively), that, in a dose-dependent manner, OTM values were associated with formaldehyde exposure levels during work hours (p=0.005) and that MN frequencies before and after the 8-hour work exposure were associated with numbers of work years (p=0.029, p<0.001, respectively). Conclusions: We found a dose-response relationship between the current formaldehyde exposure levels and DNA strand breaks and between duration of exposure and chromosome damage in the PB lymphocytes of plywood workers.

(J Occup Health 2013; 55: 284-291)

Introduction

Formaldehyde (FA) is an industrial chemical widely used in the production of resins and in their application such as in adhesives and binders used in wood products¹⁾. FA is also used in hospitals and laboratories in disinfectants, antiseptics and fixatives^{2, 3)}. Workers and professionals such as plywood workers, painters, pathologists and embalmers are constantly exposed to FA in the workplace. The general population is sporadically exposed to FA from household products such as furniture and carpeting, cosmetics, food, cigarette smoke and automobile emissions^{1, 4)}. Therefore, exposure to FA is very common in both the work and home environments, particularly in China, which has been recently summarized by Tang et al.⁴⁾.

Epidemiologic studies have increasingly linked FA exposure to cancers^5-9). In 2009, the International Agency for Research on Cancer concluded that there is sufficient evidence that FA causes nasopharyngeal cancer and leukemia¹⁰⁾. Genetic damage may be one of the early lesions in the carcinogenic process. An understanding of the genotoxic effects of FA exposure will help to elucidate the carcinogenicity of FA and could be important for developing safety programs and for preventing the adverse health effects of FA exposure.

Results from previous studies on the genotoxic effects of FA exposure on peripheral blood (PB) lymphocytes are inconsistent or even conflicting. For example, studies examining the association between duration of exposure to FA and comet assay parameters reflecting DNA strand breaks have come to different conclusions. Jiang et al.¹¹⁾ found a significant association between Olive tail moment (OTM) values in workers and years of employment, whereas Costa et al.¹²⁾ did not. Micronuclei (MN) frequencies indicating chromosome breakage or loss have been evaluated in FA-exposed populations. Ladeira et al.¹³⁾ observed an increased micronucleus frequency in the lymphocytes of workers occupationally exposed to 0.16 ppm FA (0.19 mg/m³) relative to unexposed controls. On the other hand, Ying et al.¹⁴⁾ found that the frequencies of lymphocyte micronuclei in students did not significantly increase after exposure to 0.508 mg/m³ FA over an 8-week period of anatomy classes. Moreover, debates about the correlation between chromosome damage and FA exposure levels are also found in the literature^{15, 16)}. Contradictory results were also reported for DNA-protein crosslinks (DPCs), another important genetic damage that has been extensively evaluated in FA-exposed rodents and mammalian cells in vitro^17-20). Few epidemiologic studies have investigated DPCs in human PB lymphocytes. Therefore, further investigations are needed to clarify these uncertainties regarding DNA strand breaks, chromosome breakage or loss and DPCs in the PB lymphocytes of FA-exposed individuals.

These three end points have different dynamics, persistence and implications. DNA strand breaks and DPCs are induced in cells after brief exposure to FA and are readily repaired in approximately 90 minutes and 18 hours, respectively²¹⁾. Thus, they may be transient genetic damage caused by short-term exposure to FA. At least one cell division is required for chromosome damage to form micronuclei²²⁾. Once formed, micronuclei are hard to remove from cells and may represent genetic damage accumulated over several months or even longer.

To date, no study has comprehensively examined all three types of genetic damage in PB lymphocytes of FA-exposed workers. In this study, we measured DNA strand breaks by the comet assay, micronuclei by the cytokinesis-block micronucleus assay and DPCs by the KCl-SDS assay in the PB lymphocytes of two populations from a plywood factory with FA exposure. We first compared the three genotoxic endpoints in 178 plywood workers between internal control and lower and higher exposure groups and then tested their association with FA exposure across the three groups and across three intervals of numbers of work years. We further validated the association between the genotoxic end points and FA exposure in an additional 62 plywood workers, including a comparison of their genetic damage before and after an 8-hour work shift with FA exposure.

Subjects and Methods

Study population

We recruited 178 workers from one workshop of a plywood factory in East China during the winter of 2009. An additional 62 plywood workers were recruited from another workshop of the same factory during the winter of 2011. Subjects exposed to known mutagenic agents such as radiotherapy and chemotherapy in the preceding 3 months were excluded. Structured questionnaires were administrated by trained interviewers to collect information on demographic characteristics, lifestyle factors such as smoking (yes or no) and alcohol use (yes or no), medical conditions, occupational history including job types and numbers of work years and house redecoration within the previous year (yes or no) as an estimate of major environmental exposure to FA. Individuals who had been smoking for more than 3 months were considered smokers, and those who drank more than twice a week in the last 6 months were defined as drinkers.

The 178 participants provided blood samples after their day's work, while the 62 participants provided blood samples before and after an 8-hour work exposure to FA. Written informed consent from all participants and approval from the Ethics Committee of Tongji Medical College were obtained before the study.

Exposure assessment

FA vapors in this factory came from the urea-formaldehyde glue used in plywood production. According to the production process, the workshop was generally divided into five separate workplaces, including the workplaces for (a) providing wood scraps, (b) spreading and grinding wood scraps, (c) pressing ground wood scraps with glue at high temperature and stacking up boards, (d) sanding boards and post-processing and (e) making glue. In the study of the 178 workers, we set two air-monitoring badges with a blank control in each of the five workplaces during production. The badges were connected to Gilian HFS-513A sampling pumps operating at a flow rate of 1.0 l/min for 8 hours. Replications were done the next day. FA concentrations of the samples were measured using high-performance liquid chromatography (HPLC) with UV (360 nm) detectors based on NIOSH method 2016 (Issue 2).

In the dynamic study of the 62 workers, we determined FA exposure levels of workers doing different types of jobs, including (a) providing wood scraps, (b) spreading and grinding wood scraps, (c) mixing ground wood scraps with glue, (d) pressing mixtures of wood scraps and glue at high temperature, (e) stacking up boards, (f) sanding boards, (g) post-processing, (h) making glue and (i) stoking a boiler. Air-monitoring badges were clipped near the breathing zone of 2-4 representative workers of each job type and were also connected to Gilian HFS-513A sampling pumps with a flow rate of 1.0 l/min for 8 hours. The arithmetic mean of FA concentrations was calculated as the average exposure level of the workers.

Measurement of genetic damage

Heparinized venous blood was collected for the detection of genotoxic end points. DNA strand breaks were evaluated using a comet assay in alkaline conditions as previously described²³⁾. Briefly, lymphocytes were isolated from blood samples using human lymphocyte separation medium consisting of Ficoll 400 and embedded in agarose gel on slides. After lysis (a 2-hour lysis procedure was used for the first 178 workers and an overnight lysis procedure was used for the additional 62 workers) and electrophoresis in alkaline conditions (pH=13) at 0.6 V/cm for 25 minutes, the slides were stained with 1% ethidium bromide solution and viewed with a fluorescence microscope. At least 50 lymphocytes per sample were randomly captured and analyzed in a blind fashion using Comet Assay Software Project (CASP, http://casplab.com/). One randomly selected sample was used for quality control between batches. Levels of DNA strand breaks were indicated as OTM values.

A cytokinesis-block micronucleus assay was used to detect chromosome damage according to the method described previously²⁴⁾. In brief, 0.5 ml of whole blood was cultured and stimulated with PHA for 44 hours. After cytochalasin B (6 µg/ml) was added to prevent cytokinesis, the blood was cultured for another 24 hours, and then harvested for binucleated lymphocytes, which were subsequently stained with 10% Giemsa. At least 1,000 binucleated lymphocytes per sample were randomly examined and scored by trained staff with the same criteria in a blind fashion. Levels of chromosome breakage or loss were reported as MN frequency (‰).

Zhitkovich and Costa's KCl-SDS assay was performed to detect DPCs in the PB lymphocytes²⁵⁾. Briefly, fragments of protein-bound DNA were precipitated with SDS and separated from protein-free DNA in the supernatant. The SDS precipitations were then digested with proteinase K to release DNA fragments originally bound to proteins. The levels of both protein-bound and protein-free DNA were subsequently measured with a spectrophotometer. We tested each sample in duplicate and included one positive and one negative control in each batch of the samples. The r² of the standard curves were 0.998. The DPC rate was calculated as the ratio of protein-bound DNA to total DNA (%).

Statistical analysis

MN frequencies exhibited a Poisson distribution; OTM values were normalized by natural logarithm (ln) transformation. We used one-way ANOVA to compare distribution of age and the χ² test to compare frequencies of sex, smoking status and alcohol use among the control and lower and higher exposure groups. We used ANOVA to test differences in genetic damage levels between groups of FA exposure levels with age, sex, smoking status, alcohol use and number of work years as covariances and between groups of number of work years with age, sex, smoking status, alcohol use and FA exposure levels as covariances. Trends in genetic damage levels with FA exposure levels were tested using Poisson regression models (for MN frequencies) and linear regression models (for the others) with adjustment for age, sex, smoking status, alcohol use and number of work years. Trends in genetic damage levels with number of work years were tested using Poisson regression models (for MN frequencies) and linear regression models (for the others) with adjustment for age, sex, smoking status, alcohol use and FA exposure levels. Genetic damage levels of the 62 workers after the 8-hour FA exposure were compared with those before exposure using a paired Wilcoxon test (for MN frequencies) and paired t test (for OTM values and DPC rates). Regression models for trend analyses were also used to analyze the dose-response relationship between FA exposure and genetic damage levels of the 62 workers. All statistical tests were two-tailed with a significance level of p<0.05 and performed using the Stata statistical software (Stata statistical software, release 12.0; Stata Corp, College Station, TX, USA).

Results

Ambient FA concentrations

Table 1 shows the ambient FA concentrations of the five workplaces monitored in the study of the 178 workers. We categorized the 29 workers providing wood scraps and the 53 workers spreading and grinding wood scraps into the internal control group because they had no direct work-related FA exposure. The 24 workers sanding boards and performing post-processing and the 34 workers pressing ground wood scraps with glue at high temperature and stacking up boards were categorized as the lower exposure group due to their similar relatively low exposure levels. The 38 workers making glue were categorized as the higher exposure group because their exposure level was the highest in this investigation. The mean FA exposure levels for the control and lower and higher exposure groups were 0.13, 0.68 and 1.48 mg/m³, respectively.

Table 1 Ambient formaldehyde (FA) concentrations and number of workers in the five workplaces

Workplaces	Number of workers	na	Ambient FA concentrations (mg/m3)
			Mean	Range
Providing wood scraps	29	4	0.09	0.019 to 0.164
Spreading and grinding wood scraps	53	4	0.171	0.073 to 0.252
Sanding boards and post-processing	24	4	0.603	0.455 to 0.745
Pressing ground wood scraps with glue at high temperature and stacking up boards	34	4	0.762	0.728 to 0.792
Making glue	38	4	1.48	0.914 to 2.044

^aNumber of air samples.

In the dynamic study, the FA exposure levels were 0.013 mg/m³ for providing wood scraps, 0.17 mg/m³ for spreading and grinding wood scraps, 0.24 mg/m³ for mixing ground wood scraps with glue, 0.20 mg/m³ for pressing mixtures of wood scraps and glue at high temperature, 0.46 mg/m³ for stacking up boards, 0.23 mg/m³ for sanding boards, 0.32 mg/m³ for post-processing, 0.67 mg/m³ for making glue and 0.012 mg/m³ for stoking a boiler. The mean FA exposure level of the 62 workers was 0.27 ± 0.20 mg/m³ (mean ± SD).

None of the recruited workers had been recently exposed to FA from their home environments according to the survey of house redecoration.

General characteristics of the study population

General characteristics of the 178 plywood workers in the control and two exposure groups are summarized in Table 2. The distribution of age and frequencies of sex, smoking and alcohol use were not significantly different among the three groups, although sex was close to being significant (p=0.051). The average number of work years was 2.52 ± 2.03 years (mean ± SD) for the 178 workers. Because all participants were, to some extent, exposed to FA vapors during their wor

Table 2 General characteristics and genetic damage levels of the 178 plywood workers in the control group and two exposure groups

Variables	Control group	Lower exposure group	Higher exposure group	p value	ptrende
	0.13 mg/m3 (n=82)	0.68 mg/m3 (n=58)	1.48 mg/m3 (n=38)
General characteristics
Age (year, mean ± SD)	30.54 ± 10.03	32.91 ± 8.77	32.87 ± 7.80	0.262b	—
Sex [male, n (%)]	79 (96.34)	49 (84.48)	34 (89.47)	0.051c	—
Smoking [yes, n (%)]	33 (40.24)	17 (29.31)	12 (31.58)	0.365c	—
Alcohol use [yes, n (%)]	24 (29.27)	13 (22.41)	6 (15.79)	0.257c	—
Genetic damage levels (mean ± SD)
OTM valuesa	0.67 ± 0.55	0.88 ± 0.55	1.01 ± 0.56	0.006d	0.002
MN frequencies (‰)	2.05 ± 1.72	2.02 ± 1.81	2.37 ± 1.79	0.455d	0.288
DPC rates (%)	22.73 ± 21.47	22.53 ± 20.26	20.37 ± 20.52	0.894d	0.682

^aLn-transformed for statistical inference. ^bOne-way ANOVA. ^cχ² test. ^dANOVA with age, sex, smoking status, alcohol use and number of work years as covariances. ^eCalculated using Poisson regression models (for MN frequencies) or linear regression models (for OTM values and DPC rates) with adjustment for age, sex, smoking status, alcohol use and number of work years. OTM: olive tail moment. MN: micronucleus. DPC: DNA-protein crosslinks.

k, we further divided the 178 workers into three groups of exposure duration according to the tertiles of their numbers of work years (<1, 1-3 and >3 years).

The mean age of the 62 workers in the dynamic study was 34.01 ± 10.22 years (mean ± SD). Males accounted for 82.26%, and the prevalence of smokers and drinkers was 17.74% and 30.65%, respectively. The average number of work years of the 62 workers was 2.53 ± 2.00 years (mean ± SD).

Genetic damage levels of the 178 workers

After adjustment for age, sex, smoking status, alcohol use and number of work years, OTM values were significantly higher in the lower and higher exposure groups than those in the control group (p=0.023, p=0.003, respectively) and increased with increasing levels of FA exposure (p_trend=0.002). MN frequencies and DPC rates were not significantly different between the control and exposure groups and not altered with increasing FA exposure levels in this population (Table 2). However, we found that MN frequencies were significantly higher in the 1-3 years and >3 years groups than those in the <1 year group (p<0.001 for both) and increased in proportion to the number of work years (p_trend<0.001). We also found that OTM values and DPC rates tended to increase as the number of work years increased, but the increase was not statistically significant (Table 3).

Table 3 Genetic damage levels of the 178 plywood workers in the three groups based on the number of work years

Variables	Number of work years			p valueb	ptrendc
	<1 year (n=57)	1;3 years (n=64)	>3 years (n=57)
OTM values (mean ± SD)a	0.76 ± 0.56	0.73 ± 0.59	0.99 ± 0.52	0.131	0.059
MN frequencies (‰, mean ± SD)	1.02 ± 1.10	2.25 ± 1.56	2.90 ± 1.96	<0.001	<0.001
DPC rates (%, mean ± SD)	19.34 ± 20.77	22.10 ± 20.98	25.06 ± 20.57	0.577	0.376

^aLn-transformed for statistical inference. ^bANOVA with age, sex, smoking status, alcohol use and formaldehyde (FA) exposure levels as covariances. ^cCalculated using Poisson regression models (for MN frequencies) or linear regression models (for OTM values and DPC rates) with adjustment for age, sex, smoking status, alcohol use and FA exposure levels. OTM: olive tail moment. MN: micronucleus. DPC: DNA-protein crosslinks.

Genetic damage levels of the 62 workers

In the dynamic study of the 62 workers, OTM values and DPC rates were significantly increased by the end of an 8-hour occupational FA exposure compared with OTM values and DPC rates before the exposure (p<0.001, p=0.019, respectively), whereas MN frequencies before and after the exposure were not significantly different (Table 4). The increase in OTM values, but not in DPC rates, was associated with the FA exposure levels during work hours in a dose-dependent manner (β=4.27, p=0.005) (Table 5).

Table 4 Genetic damage levels of the 62 plywood workers before and after an 8-hour FA exposure

Variables	Before the exposure			After the exposure			p value
	na	Mean ± SD	Range	na	Mean ± SD	Range
OTM values	60	1.47 ± 0.72	0.03 to 3.38	62	2.30 ± 1.28	0.42 to 5.72	< 0.001b
MN frequencies (‰)	62	2.29 ± 1.21	1 to 6	62	2.29 ± 1.65	1 to 9	0.754c
DPC rates (%)	62	27.22 ± 10.07	11.11 to 53.31	60	31.68 ± 14.19	3.81 to 56.60	0.019b

^aThe numbers of observations vary because of missing values. ^bPaired t test. ^cPaired Wilcoxon test. OTM: olive tail moment. MN: micronucleus. DPC: DNA-protein crosslinks.

Table 5 Association of formaldehyde (FA) exposure levels and number of work years with genetic damage levels of the 62 plywood workers before and after an 8-hour FA exposure

Dependent variables	Association with FA exposure levels		Association with number of work years
	β coefficient (95% CI)	p valueb	β coefficient (95% CI)	p valuec
OTM values
Before the exposure	−0.69 (−2.11, 0.73)	0.335	−0.06 (−0.13, 0.02)	0.132
After the exposure	3.64 (1.36, 5.92)	0.002	0.10 (−0.02, 0.22)	0.088
Differencea	4.27 (1.36, 7.18)	0.005	0.15 (0.00, 0.30)	0.057
MN frequencies (‰)
Before the exposure	0.73 (−0.46, 1.92)	0.227	0.06 (0.01, 0.11)	0.029
After the exposure	−0.01 (−1.38, 1.35)	0.985	0.09 (0.04, 0.14)	< 0.001
Differencea	−1.59 (−5.03, 1.85)	0.358	0.15 (−0.03, 0.33)	0.106
DPC rates (%)
Before the exposure	1.70 (−17.84, 21.24)	0.862	−0.83 (−1.86, 0.19)	0.109
After the exposure	−6.04 (−31.23, 19.15)	0.633	0.14 (−1.18 1.46)	0.831
Differencea	−11.40 (−39.56, 16.76)	0.42	0.86 (−0.62, 2.33)	0.249

CI, confidence interval. ^aAfter the exposure - before the exposure. ^bCalculated using Poisson regression models (for MN frequencies) or linear regression models (for OTM values and DPC rates) with adjustment for age, sex, smoking status, alcohol use and number of work years. ^cCalculated using Poisson regression models (for MN frequencies) or linear regression models (for OTM values and DPC rates) with adjustment for age, sex, smoking status, alcohol use and FA exposure levels. OTM: olive tail moment. MN: micronucleus. DPC: DNA-protein crosslinks.

In addition, we found that OTM values after the 8-hour FA exposure, but not before the exposure, were associated with FA exposure levels (β=3.64, p=0.002). MN frequencies and DPC rates before or after the exposure were not significantly associated with FA exposure levels. We also found that MN frequencies before and after the workplace exposure to FA were significantly associated with the number of work years (β=0.06, p=0.029; β=0.09, p<0.001, respectively). However, OTM values and DPC rates before or after the work exposure were not associated with the number of work years (Table 5).

Discussion

Our findings from this study provide clear evidence that FA exposure is associated with genetic damage in the PB lymphocytes of plywood workers and further indicate that different types of genetic damage may be attributed to different exposure levels and durations.

In agreement with a previous study on plywood workers¹¹⁾, we found a dose-dependent increase in DNA strand breaks in PB lymphocytes with increasing FA exposure levels. Unlike the Jiang et al. study¹¹⁾ but consistent with the study by Costa et al.¹²⁾, we did not find a significant association between OTM values and number of work years. Our findings further suggest that DNA strand breaks in PB lymphocytes may be induced by recent FA exposure, as shown by the increased OTM values after occupational exposure to FA for 8 hours in the dynamic study. The association of FA exposure levels with OTM values after, but not before the workday exposure, suggests that repair of the majority of DNA lesions in the PB lymphocytes occurs during the ∼16 hours period between work shifts. An in vitro study suggested that FA-induced DNA strand breaks would be repaired within 90 minutes after FA exposure ended and could not accumulate in cells²¹⁾.

Chromosome lesions may accumulate in PB lymphocytes of plywood workers through misrepair of DNA lesions. After adjustment for age and other confounding factors, we found that MN frequencies from samples taken before (67 worker study) and after (67 and 178 worker studies) the workday FA exposure were significantly increased with the number of work years, consistent with the findings of Costa et al.¹²⁾ and Jiang et al.¹¹⁾ in 48 pathology/anatomy workers and 151 plywood workers, respectively. However, in contrast to the findings of the two earlier studies by Costa et al.¹⁵⁾ and Jiang et al.¹¹⁾, we did not find a significant association between the MN frequencies of the PB lymphocytes (before or after the work exposure) and FA exposure levels. Consistent with our results, Orsière et al.¹⁶⁾ showed that MN frequencies were not related to personal sampling data for FA, although the MN frequencies were significantly higher in the exposed group than in the control group. Taken together, our findings suggest that the chromosome damage levels of the PB lymphocytes might reflect cumulative damage acquired through frequent exposure over a longer period (years) rather than recent FA exposure. It remains to be determined whether this damage arises in long-lived T cells (the predominant lymphocyte type in peripheral blood) or in hematopoietic progenitor cells.

In reports by Liu et al.²¹⁾ and Frenzilli et al.²⁶⁾, FA was proposed to mainly induce DNA strand breaks at low levels and DPCs at high levels. In our study, the rates of DPC formation were not associated with FA exposure levels, and although DPC rates significantly increased after the work exposure compared with those before the work exposure (which might be just a chance finding, considering physical activities might potentially elevate crosslink between DNA and histones or other proteins), the increase was not associated with FA exposure levels during work hours. Together with the other finding that DPC rates were not associated with the number of work years, we concluded that DPCs were not the major genetic damage in the PB lymphocytes of workers exposed to the current FA concentrations. Shaham et al.²⁷⁾ found that FA-exposed technicians and physicians had significantly higher DPC rates in peripheral lymphocytes compared with unexposed workers and that technicians with higher FA exposure levels had significantly higher DPC rates compared with physicians with lower FA exposure levels. However, the environmental FA concentrations in Shaham's study ranged from 2.8 to 3.1 ppm (3.44-3.81 mg/m³), which was far higher than those in our study. Thus, the different outcomes of the two studies may be due to the different exposure levels.

We assessed the influence of major confounding factors such as age, sex, smoking status and alcohol use on different genotoxic endpoints. We found that OTM values and MN frequencies of the PB lymphocytes significantly increased with age after adjustment for FA exposure levels and number of work years. It has been postulated that the increase in spontaneous chromosomal instability and the decrease in efficiency of DNA repair mechanisms, which occur in aging, may lead to the accumulation of genetic damage in PB lymphocytes^{16, 28)}. Sex has been proposed as a confounding factor in the evaluation of genetic lesions because the X-chromosome might be more vulnerable to aneugenic events²⁹⁾. In addition, tobacco smoke contains a certain amount of FA and other mutagenic or carcinogenic substances³⁰⁾, and alcohol may share common metabolic pathways with FA³¹⁾; hence they should also be considered as confounding factors in the present study. However, we found no significant difference in genetic damage levels between males and females, smokers and nonsmokers, and drinkers and nondrinkers. The relatively low number of females, smokers and drinkers may have weakened the influence of these factors. The influence of house redecoration as a major environmental FA exposure could not be estimated in this study because none of the participants had reported house decoration in the preceding year.

Our study has several strengths. We systematically evaluated three types of genetic damage including DNA strand breaks, chromosome breakage or loss and DPCs in the PB lymphocytes of FA-exposed workers before and after an 8-hour occupational exposure. Further, our study was able to examine the association of these outcomes with FA exposure levels and with the number of work years. Our findings revealed a dose-response relationship between FA exposure levels and DNA strand breaks, as well as between the number of work years and chromosome breakage or loss in PB lymphocytes. Our findings also suggest that DNA strand breaks might reflect recently acquired damage, while chromosome damage might reflect cumulative effects from frequently recurring exposure to FA.

The limitations of the present study should also be addressed. First, genotoxic end points, particularly DPC rates, in PB lymphocytes of workers with higher FA exposure levels than 1.48 mg/m³, the highest FA exposure level of the plywood workers in this study, should be further evaluated. Second, determination of personal FA exposure levels of workers may provide more accurate evaluation of the dose-response relationship between FA exposure and genetic damage than estimation of exposure levels based on workplaces or job types. Third, although smoking and house redecoration were assessed as environmental FA exposures in this study, exposure from diet, cosmetics and automobile emissions, which has been thoroughly reviewed by Tang et al.⁴⁾, was not considered; therefore, we cannot exclude the possibility that our results might be biased by these potentially confounding factors.

In conclusion, we found a dose-response relationship between current FA exposure levels and DNA strand breaks, as well as between duration of FA exposure and chromosome breakage or loss in the PB lymphocytes of plywood workers. Findings from this study have helped to clarify previous uncertainties about the association between FA exposure and different types of genetic damage in human PB lymphocytes.

Funds: This work was supported by the National Key Technologies R&D Program of China [2006BAI06B02] and the National Basic Research Program of China (973 program) [2011CB503800].

Competing interests: The authors declare they have no actual or potential competing interests.

Acknowledgments: We thank the members of Shandong Academy of Occupational Health and Occupational Medicine (Ji'nan, China) for assistance with sample collection and instrumental support and L. Zhang and C. McHale for their critical review and comments on this article.

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