Abstract
Workers within the megacity of Los Angeles are exposed to significant amounts of airborne particulate matter (PM) during their daily commutes, which often exceed 30–60 minutes each way. Chemical species present in roadway and railway PM, including Benzo[a]pyrene (BaP) and hexavalent chromium (Cr6+), present substantial cancer and non-cancer health risks. In the current study, PM samples were collected and quantitatively speciated along five major commuter routes, including the METRO red line (subway) and gold line (light rail), the I-110 and I-710 freeways, and high-density surface streets (Sunset and Wilshire Boulevards). Using these concentration data, along with cancer potency (CP) and Reference Dosage (RfD) factors obtained from the United States Environmental Protection Agency (USEPA) and California's Office of Environmental Health Hazard Assessment (OEHHA), cancer and non-cancer health risks were calculated. In contrast to previous research indicating that Polycyclic Aromatic Hydrocarbon (PAH) components of Los Angeles roadway PM (e.g., along the I-710 freeway) led to the greatest cancer risk, the current analysis reveals that exposure to carcinogenic transition metals, particularly hexavalent chromium, which are especially prevalent along the METRO red line, results in the greatest cancer and non-cancer health risks. Based on these data, the best option for commuters is to use above-ground light-rail transportation, which allows for reduced exposure to both traffic-generated PAHs and railway-related metals.
Highlights
In megacities such as Los Angeles, Seoul, Tokyo, Moscow, Tehran, and São Paulo, whose populations exceed 10 million inhabitants, commuters rely heavily on public transportation, including bus, subway and light-rail transit systems, as well as personal vehicles
In contrast to previous research indicating that Polycyclic Aromatic Hydrocarbon (PAH) components of Los Angeles roadway particulate matter (PM) led to the greatest cancer risk, the current analysis reveals that exposure to carcinogenic transition metals, hexavalent chromium, which are especially prevalent along the METRO red line, results in the greatest cancer and non-cancer health risks
The assumption made by Chillrud et al (2004) that all detected chromium is hexavalent chromium, an assumption we make in this paper, is reasonable given the high temperatures in which railway dust is formed, and allows us to determine a more conservative health risk estimate
Summary
In megacities such as Los Angeles, Seoul, Tokyo, Moscow, Tehran, and São Paulo, whose populations exceed 10 million inhabitants, commuters rely heavily on public transportation, including bus, subway and light-rail transit systems, as well as personal vehicles. Expose the public to varying degrees of air pollution during their daily commute, and the concomitant health risks can be significant. Airborne particulate matter (PM) is considered one of the most toxic forms of air pollution to which commuters are typically exposed, in both roadway and railway environments. Two classes of compounds commonly found in the airborne PM to which commuters are exposed, polycyclic aromatic hydrocarbons (PAHs) and transition metals, contain species known to be carcinogens and/or pose chronic health risks (Harrison et al, 2004). Polycyclic Aromatic Hydrocarbons (PAHs), such as Benzo[a]pyrene (BaP), are formed during incomplete fossil fuel combustion and are typically found in the PM emitted by vehicles. Transition metals, including nickel (Ni), iron (Fe), and hexavalent chromium (Cr6+), often result from the friction-induced wear of railway components, especially steel rails and cables, and represent a significant fraction of light-rail and subway PM emissions (Chillrud et al, 2004; Seaton et al, 2005)
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