Health-based Risk Assessment Of 1-Bromopropane

1-Bromopropane (1-BP) is currently under review per the US EPA Significant New Alternatives Policy program in identification of substitutes for ozone-depleting substances. 1-BP is being considered as a possible replacement for chlorofluorocarbons (HCFC), 1,1,1-trichloroethane and methyl chloroform for nonaerosol solvent cleaning of metals and electronics and for adhesives, coatings, aerosol propellant, and solvent applications. Since late '90s, the production and use of 1-BP are increasing rapidly.1-Bromopropane is a neuro and reproductive toxicant in animals and humans, however, the toxic mechenism at gene level remains unknown. The present study explored the change of gene expression profile in male F344/N rats after exposure to 1-BP to clarify its possible toxic mechenism at mRNA level. Twelve F344/NSIc male rats were randomly divided into two groups of 6 each. Rats were exposed to either fresh air or 5030 mg/m3 1-BP through inhalation for 8 h. Rats were sacrificed and testes and brains were removed at 16 h after exposure. Global changes in gene expression were determined by microarray analysis using rat chip followed by Real-time PCR validation. Results showed that among the 5082 genes and ESTs in the genital chip,62 genes were down-regulated and 3 genes were up-regulated by 1-BP, which include synthetic sex hormone-related genes cytochrome P450 aromatase (Cypl9a), glutathione S-transferase (GSTT1), creatine kinase (Ckb), myelin and lymphocyte protein (Mal) and S100 calcium-binding protein (S100a4). Classification analysis revealed that the majority of gene changes was invoved protein/lipid metabolism, followed by the stress-associated defense response. Real-time PCR confirmed the down-regulation of Cyp19a, GSTT1, Mal and S100a4 genes. The study demonstrate that acute high-dose exposure to 1-BP causes the down-regulation of Cypl9a, S100a4, GSTT1 and Mal mRNA in rats. This altered gene profiles might reflect the toxic mechanism which suggested that 1-BP disrupt the biological metabolism and endocrine balance.So far, very limited human data regarding 1-BP exposure could be found. This study investigated the health effects of 1-BP on exposed workers in four 1-BP manufacturing plants. Workers were interviewed with questionnaire and examined with neurobehavioral core test battery, nerve conduction velocity tests of nervus tibialis and nervus suralis, vibration sensation test, hematological and biochemical tests. Ambient 1-BP concentration was measured with detection tube, and time-weighed average levels of individual workers were estimated with passive samplers. Results showed that 1-BP concentration in the plants ranged 0-402.40 mg/m3 (0-80ppm),Geomean 32.19mg/m3(6.40ppm). Time-weighted average exposure levels (TWA-8h) ranged 0.35-535.19 mg/m3 (0.07-106.40ppm),Geomean 14.08mg/m3 (2.80ppm). Compared with the control group,1-BP exposed workers showed reduced motor nerve conduction velocity (44.8±8.7m/s) and sensory nerve conduction velocity(45.5±4.9m/s), prolonged distal latency(7.5±2.1ms), reduced toe vibration perception, and altered neurobehavior parameters (POMS Vigor, Tension, Anxiety, Confusion) significantly (p<0.05). As to hematological and biochemical indicators, the exposed workers showed decreased white blood cell count (21.0±5.5×103/μL), red blood cell count (3.9±0.4×106/μL), hemoglobin (121.1±14.5 g/L) and creatine kinase (82.0±27.5 IU/L) (p <0.05), and increased serum total protein (8.0±0.5 g/dL), lactate dehydrogenase (335.2±356.6 IU/L), thyroid-stimulating hormone (3.6±2.3μIU/mL) and follicle-stimulating hormone levels (18.7±24.4mIU/mL) (p<0.05). The study indicate that 1-BP exposure may affect peripheral nerves and central nervous system, and lead to abnormal hematological and biomedical indicators.Our microarray study showed that 1-BP exposure could induce the GSTT1 gene expression significantly. GSTM1 and GSTT1 polymorphism phenomena exist commonly in population. Study on the relationship between these two polymorphism and the workers' susceptibility to 1-BP neurotoxicity will play an important role in the prevention and control of occupational hazards as well as the protection of susceptible workers.The present study explored the association between GSTTl and GSTM1 Gene polymorphism and nerve conduction velocity in workers exposed to 1-BP. Neuroelectrophysiological studies were carried out on 116 1-BP exposed and 133 non-exposed workers. The polymorphism of GSTT1,and GSTM1 were analyzed by allele specific amplification(ASA) method. Result showed that GSTT1 gene polymorphism and exposure had an interact effect on sensory nerve conduction velocity(SCV) and distal latency(DL). Workers with GSTT1 gene null type in exposed group had a slower SCV (40.1±5.3m/s) and longer DL (8.3±2.0ms) compared with those with GSTT1 gene normal type in non-exposed group [SCV(45.3±5.0)m/s, DL(6.3±1.3)ms](P<0.05). There is no evidence that GSTM1 gene polymorphism had effect on nerve conduction velocity of these workers. The study suggest that GSTT1 gene polymorphism might be associated with the susceptibility to neurotoxicity of 1-BP.Based on the data of workers, the study calculated the 1-BP toxicity benchmark dose. USEPA BMDS 1.4.1c software was applied to calculate 1-BP benchmark dose (BMD) and its 95% lower limit (BMDL). BMD calculation based on DL as 1-BP neurotoxic effect endpoint showed that TWA-8h of the BMD value and BMDL value were 36.77 mg/m (7.31 ppm)and 21.73 mg/m3(4.32 ppm). BMD calculation based on serum CK as 1-BP biochemical toxic effect endpoint showed the BMD value and BMDL value were 88.28 mg/m3 (17.55 ppm) and 44.82 mg/m3 (8.91 ppm). BMD calculation based on serum TSH as 1-BP biochemical toxic effect endpoint showed the BMD value and BMDL value were 47.23 mg/m3 (9.39 ppm)和31.54 mg/m3 (6.27 ppm).Health-based risk assessment of 1-BP was conducted on the basis of the comprehensive analysis of above animal/human studies and literatures. It is assumed that 1-BP is not a chemical widespread in the environment and occupational exposure is considered the main route of its exposure to humans. The risk identification of 1-BP was mainly dependent on animal data as very few evidence in humans are available. There is sufficient evidence in rats that 1-BP causes dose-dependent neurotoxicity, developmental and reproductive toxicity. There is insufficient evidence in animals that 1-BP causes hemotoxicity, genetic toxicity and carcinogenicity. A benchmark concentration 95th percentile lower confidence limit of 1,534.00 mg/m3(305 ppm) was identified from a rat inhalation developmental toxicity.. A LOAEC of 1,257.00 mg/m3(250 ppm) for female reproduction (NOAEC= 503.00 mg/m3[100 ppm] was identified from an inhalation, two-generation reproductive toxicity study.. A LOAEC of 1,006.00 mg/m3(200 ppm) for male reproduction (NOAEC-503.00 mg/m3[100 ppm] was identified from other two reproductive toxicity studies. Available human data are insufficient to draw conclusions on the reproductive or developmental toxicity of 1-BP. Case reports of 1-BP intoxication and the occupational epidemiology studies confirmed the neurotoxicity of 1-BP in humans.So far there is no occupational exposure limit of 1-BP in China. Based on the above BMD calculations in workers and with reference to the limits set by EPA and ACGIH, the present study is recmmending an TWA-8h exposure limit of 25.15 mg/m3 (5 ppm). It is recommended that 1-BP exposed workers undergo health examination on neurological functions and blood tests regularly, for an early detection of occupational health hazards.In summary, the present study conducted 1-BP occupational epidemiology investigation in China, and reported that workers exposed to 1-BP showed neurological abnormalities manifested as enlongated distal motor latency (DL) and reduced vibration sense, and serum biochemical changes manifested as elevated CK elevated and decreased TSH levels. The BMD calculated with DL as toxic endpoint and this provided a central estimate for the BMD of 36.77 mg/m3 (7.31 ppm) and BMDL of 21.73 mg/m3 (4.32 ppm). The BMD calculated with serum CK level as toxic endpoint and this provided a central estimate for the BMD of 47.23 mg/m3 (9.39 ppm) and BMDL of 31.54 mg/m3 (6.27 ppm). The present study is recmmending an TWA-8h exposure limit of 25.15 mg/m3 (5 ppm) as the 1-BP occupational exposure limit in China. The study suggest that GSTT1 gene polymorphism might be associated with the susceptibility to neurotoxicity of 1-BP. The study also suggest that the toxic mechanism of 1-BP involve the disruption to the biological metabolism and endocrine balance.