Characterization of occupational exposure to air pollutants during asphalt milling and paving

Asphalt is frequently used as road pavement and consists of bitumen as a binder, and fillers. Bitumen consists of a complex mixture of hydrocarbons, where a minor component is polycyclic aromatic hydrocarbons (PAHs). Many PAHs are classified as carcinogenic to humans. Bitumen fumes from road paving have been classified as possibly carcinogenic. Paving and milling are open processes generating asphalt fumes, mechanically generated dust particulate matter and diesel exhaust, which the asphalt workers are exposed to. Ultrafine particles (UFPs) are present in both asphalt fumes and diesel exhaust. The aim was to characterize occupational exposure of milling and road paving with a novel multi-metric approach by using real-time monitors and offline methods. Additional aims were to monitor asphalt workers' skin contamination of PAHs by skin wiping, and to biologically monitor their systemic exposure to PAH in urine.

Personal exposure measurements of lung deposited surface area (LDSA), particle number concentration (PNC), particulate mass (PM0.3), average particle size, organic carbon (OC), elemental carbon (EC), equivalent black carbon, 16 US Environmental Protection Agency (EPA) PAHs, and nitrogen dioxide (NO2) were performed on millers and pavers in a field study. Skin wipe samples (palm) and urine samples were collected before and after workshifts and were analysed for PAH and PAH metabolites, respectively. Repeated self-administered samplings of 16 US EPA PAHs and NO2 were performed twice by the millers and pavers.

The pavers had the highest average exposure to all exposure metrics, except for OC and NO2. Their geometric mean (GM) exposures to PNC and LDSA were 31,000/cm3 and 80 µm2/cm3, respectively. The GM exposure to 16 US EPA PAHs, OC, EC, and NO2 were 0.29, 21, 0.75, and 31 µg/m3, respectively. The millers' GM exposures to PNC and LDSA were 29,000/cm3 and 67 µm2/cm3, respectively. Their GM exposure to 16 US EPA PAHs, OC, EC, and NO2 were 0.053, 40, 0.40, and 83 µg/m3, respectively. The self-administrated sampling of 16 US EPA PAH and NO2 showed that the exposures were in the same range as in the field study, increasing the validity of the results. Pavers showed significantly higher levels of PAH on the palm after the workshift compared with millers. Millers showed higher levels of benzo[a]pyrene on their palm after the workshift compared with pavers. The urinary levels of PAH metabolites were significantly increased in pavers after the workshift.

This study showed that millers and pavers were exposed to airborne 16 US EPA PAHs, UFPs, OC, and diesel exhaust. With a study design that involved repeated exposure measurements for each participant, more accurate exposure characterization and assessment of PAHs and NO2 were obtained. By using portable aerosol monitors, valuable exposure data for novel metrics, including UFPs, could be obtained. Operators of, eg, rollers and milling machines were exposed to multiple peak exposures during the workshift. Millers were exposed to somewhat elevated levels of the carcinogenic particulate PAHs. As biomonitoring generally is measuring metabolites of gaseous and intermediate molecular mass PAHs, particulate PAH exposure could not be detected. Air and skin exposure measurements were vital in order to detect this exposure. Recommendations for reducing occupational exposure are proposed.