Evaluation of telomere length and genotoxicity among asphalt associated workers

Highlights

• IARC classified occupational exposure to asphalt as possibly carcinogenic to humans.

• In India, asphalt workers do not use a adequate personal protective measures to reduce exposure.

• Increased biomarkers of genetic damage were seen in asphalt workers compared with controls.

• The observed genetic damage in asphalt workers is associated with possibility of increased malignancy risk.

• Asphalt workers should be provided PPE to reduce risk for severe adverse health effects.

Abstract

There are contradictory reports about bitumen exposure and malignancy risk worldwide. Also, the evidence for genotoxicity risk among workers occupationally exposed to asphalt is insufficient. The study intended to evaluate particulate matter 10 (PM₁₀) at the workplace and biomarkers of genotoxicity effects among a group of asphalt workers in and around Bangalore, India. This study involved a total of 107 participants (54 exposed group and 53 unexposed control group). To evaluate the genotoxicity, the urinary 8-OHdG and relative telomere length as oxidative damage while micronucleus (MN) assay for cytogenetic damage was carried out during the study. The majority of workers have reported health complaints and 57.4% of them were not using any personal protective equipments (PPE’s). The level of PM₁₀ detected was 104 ± 9.5 μg/m³ and 619 ± 22.7 μg/m³ in the road paving and asphalt mixing sites respectively. The biomonitoring study observed a highly significant (p = <0.001) increase in the level of 8-hydroxy-2-deoxyguanosine (8-OHdG) in the exposed group (23.17 ± 8.65 ng/mg creatinine) compared to the control (13.6 ± 7.12 ng/mg creatinine), revealed age significant associated and non-smoking borderline significant associated for oxidative stress. The relative telomere length (TL) analysis revealed its highly significant (p = 0.004) reduction in the exposed group, adjusted mean 0.95 (95% CI 0.83–1.07) compared to the control 1.06 (95% CI 0.91–1.26). The job category (p = 0.028), non-smoking (p = 0.026), and tobacco chewing (p = 0.013) were associated with reduced relative TL in the asphalt exposed group. In cytogenotoxicity analysis, the mean micronucleus (MN) frequency per 100 cells in the exposed group (26.46 ± 19.8) was significantly (p = <0.001) increased over the control group (8.56 ± 7.18). Neither smoking habit nor age appeared to influence the MN frequencies in either group. In the present study, we have demonstrated genetic damage in workers occupationally exposed to asphalt and particulate matter, raising concern for an increased risk of malignancy in these workers.

Introduction

Asphalt workers are one of the major occupational groups being exposed to a variety of chemical, biological, or physical (e.g. noise, heat, and radiation) agents throughout their work-life [1]. The bitumen asphalt preparation and road paving tasks chronically expose workers to polycyclic aromatic hydrocarbons (PAHs) both by inhalation and dermal contact [2]. Asphalt is mixtures of bitumen, small stones, sand, and filler primarily used for road paving. Asphalt is also known as bitumen is a dark, semisolid form of petroleum product, a common binder or glue used in road construction and its emulsions are often sprayed onto the road surface before paving [3]. Bitumen contains several chemicals, including polycyclic aromatic hydrocarbons (PAHs), volatile organic compounds (VOCs), and metals. Exposure to PM, VOCs, and various polycyclic aromatic compounds is common among bitumen workers and the level of exposure varies at different steps of processing and road paving. About 93% of inhalable dust emitted during road paving consists of organic particulate matter [4]. The PAHs associated particles with aerodynamic diameters below 10 μm (regarded as PM₁₀) are respirable and can deposit in the human respiratory system [5]. As per WHO, particulate matter is one of the most common indoor and outdoor air pollutants [6].

During paving, hot application of asphalt complex mixtures of aerosols and vapors are emitted which contain carcinogens such as PAHs and other compounds [7,8]. Also, the earlier study confirmed the risk of carcinogen exposure (Benzo(a)pyrene) and carcinogenic hazards among asphalt paving workers [9]. Recently, the International Agency for Research on Cancer (IARC) classified occupational exposures to conventional asphalt and its emissions during road paving as group 2B (possibly carcinogenic to humans) [10]. It has been shown that exposure to asphalt fumes causes DNA damage [1]. The 8-OHdG is considered as major DNA damage caused by reactive oxygen species (ROS) and its formation occurs due to the attack of carbon 8 (C8) in guanine by hydroxyl radicals [11]. The early steps in the development of cancer, mutation, and aging are the result of oxidative damage to the DNA in cells [12].

Telomeres are the tandemly repeated G rich (5′ TTAGGG 3′) nucleotide protein structures at the terminal regions of eukaryotic chromosomes that protect the genetic material of the cells and its dysfunction leads to carcinogenesis in human. [13,14]. Telomere length serves as a biological clock of the living organism and in humans, its length decreases with age. During the normal cellular process, the small portion of telomeric DNA is lost with each cell division [15]. While additional attrition of telomere length shown to be accelerated by oxidative stress, deficient DNA repair capacity [16], environmental and occupational exposure to traffic-related air pollution (i.e. PM, black carbon (BC), benzene and toluene), PAHs, N-nitrosamines, pesticides, lead exposure in car mechanical workshops and hazardous waste exposures [17,18]. The chromosomal end containing telomeric DNA regions is sensitive to the reactive oxygen species (ROS) damage [19]. It has been suggested that telomere attrition might be used for biomonitoring purposes and telomere length may also work as a marker of susceptibility [20].

The first barrier to the inhalation or ingestion of carcinogens is buccal epithelial cells. These cells are capable of metabolizing carcinogens to reactive products. About 90% of human cancers originate from epithelial cells and hence represent a preferred target for early genotoxic events induced by carcinogenic agents [21]. Micronuclei (MN) assay in exfoliated buccal cells is a useful and minimally invasive method for monitoring genetic damage in humans [22]. The earlier studies suggest that MNs and altered telomere length are biomarkers associated with increased cancer risk [[23], [24], [25], [26]]. According to epidemiological studies, there is a poor correlation between bitumen exposure and cancer risk [27,28] but the effects of bitumen exposure on health shouldn’t be ignored.

Therefore, the present study intended to assess the genotoxic hazards associated with exposure to asphalt fumes, particulate matter, engine exhaust, and passing traffic exposure by monitoring workplace ambient PM₁₀, oxidative stress (8-OHdG), relative TL, and cytogenetic damage (MN) among asphalt related workers. As per our knowledge, this is the very first study evaluating the effect on telomere length amongst Indian asphalt workers.