A study by a team from the California Air Resources Board (ARB) has found that the collective cancer risk from exposure to seven toxic air contaminant (TACs) has declined 76% during the period from 1990 to 2012, and linked that result from air quality regulations targeting these TACs. The study appears in the ACS journal Environmental Science & Technology.
Of the seven TACs, diesel particulate matter (DPM) is the most important; DPM is emitted mainly from trucks and buses and is responsible for most of the airborne cancer risk in California, according to ARB. However, in the study DPM is not measured directly. Based on a novel surrogate method, DPM concentrations declined 68% during the period, even though the state’s population increased 31%; diesel vehicle-miles-traveled increased 81%; and the gross state product (GSP) increased 74%.
|Credit: ACS, Propper et al. Click to enlarge.|
The nearly 70% drop in DPM coincided with actions taken over the years, beginning in the 1990s, to reduce diesel emissions. In the 1990s, California adopted a reformulated diesel fuel program; started a heavy-duty diesel truck roadside inspection program; implemented particle pollution standards for urban transit buses; and established standards for off-road diesel engines. In 2006, California began requiring ultra-low-sulfur diesel fuel.
|For the study, the researchers estimated ambient DPM concentrations using oxides of nitrogen as a surrogate, combined with scaling factors derived from the ratio of DPM emissions to total NOx emissions as obtained from the ARB inventory.|
|Basin-wide annual average ambient NOx concentrations (1990−2012) were calculated using hourly average measurements from over 200 monitoring sites.|
|The following assumptions were made: (1) ambient DPM concentrations are proportional to NOx concentrations in each air basin; (2) emissions are well-mixed on basin-wide time scales, and (3) background NOx concentrations can be neglected.|
|Full details on the methodology are presented in the supplementary materials for the paper.|
Following the establishment of California’s statewide Truck and Bus Rule in 2008, California began requiring diesel particulate filters on trucks, reducing diesel particulate matter from the exhaust gas of diesel engines.
Based on monitoring data, concentrations of benzene; 1,3-butadiene; perchloroethylene; and hexavalent chromium declined 88–94%. (The reduction of benzene and 1,3-butadiene was largely the result of California gasoline reformulation in 1996, ARB said.) Also, the ambient and emissions trends for each of these four TACs were similar. Formaldehyde and acetaldehyde, which are formed in the air photochemically from volatile organic compounds (VOCs), declined only 20–21%.
The paper makes clear that further significant reduction in cancer risk to California residents is expected to continue as a result of continued implementation of air toxic controls. Such controls are part of broader statewide transportation initiatives, including the Truck and Bus Rule and more than a dozen rules focused on diesel equipment serving ports and railyards. Neighborhoods in freight corridors, including those near ports, will especially benefit.
ARB regulations have reduced air toxics emissions from vehicles and their fuels, from stationary sources and from consumer products since the mid-1980s. In response to public concern, the California Legislature passed the Toxic Air Contaminant Identification and Control Act in 1984. Since then, ARB has implemented regulations to limit TAC emissions. In 1987, the California Legislature passed the Air Toxics “Hot Spots” Information and Assessment Act, which requires businesses to reduce risks from exposure to emitted TACs.
Ralph Propper, Patrick Wong, Son Bui, Jeff Austin, William Vance, Álvaro Alvarado, Bart Croes, and Dongmin Luo (2015) “Ambient and Emission Trends of Toxic Air Contaminants in California” Environmental Science & Technology doi: 10.1021/acs.est.5b02766