Boxplot of average PM2.5, BC, UFP, and LDSA [lung deposited surface area] concentrations for each commute mode. The boxes represent the interquartile range (IQR, 25th to 75th percentile), whiskers represent 10th and 90th percentile, and red circle is sample average. Ham et al. Click to enlarge.

Average concentrations of particulate matter and black carbon were statistically similar for cars, buses, and bicycle trips, and in between the levels found in the two types of train commutes. Since the average car and public transport trips are much faster than bicycle trips, they may offer shorter exposure durations; however, cycling has significant health benefits.

The study measured air pollution exposures to harmful traffic-related air pollutants during a variety of travel modes to and from CARB headquarters in downtown Sacramento.

The researchers developed an innovative air pollution measurement backpack with state-of-the-science pollution sensors, and recruited volunteers to collect data during their daily commute trips. Commute modes included travel by car, bus, light rail, train, and bicycle.

The study also compared air pollution exposure per mile for each mode, a useful metric for people to use when selecting a travel mode that offers the lowest air pollution exposure for their individual commute.

Light rail commutes had the lowest average exposure per mile for all measured pollutants, and car trips experienced marginally higher per mile exposure, whereas train commutes with older diesel technologies experienced the largest exposure per mile of all of the motorized transportation commute modes. The study also offers advice for reducing exposure to air pollution during commute trips:

• Car travelers can reduce their personal exposure to PM_2.5, ultrafine particles, and black carbon by up to 75% by operating the air conditioner on recirculate mode.

• Bicycle commuters can reduce exposures by between 15 and 75% by choosing dedicated bicycle pathways away from traffic sources.

• Older technology diesel-powered train commute trips where the locomotive engine was pushing the rail cars experienced up to 90% lower ultrafine particle concentrations than ones where the locomotive engine was pulling the cars.

The ARB team found that UFP concentrations during bus, bicycling, and train commutes were 1.6–5.3 times greater than personal vehicle commutes, while light rail commutes had 30% lower UFP concentrations than personal vehicle commutes. The largest exposure per mile occurred during bicycle commutes with PM_2.5, BC, and UFP exposures of 1.312 μg/mile, 0.097 μg/mile, and 3.0 × 109 particles/mile, respectively.

Train commutes experienced the largest exposure per mile of all of the combustion-derived transportation commute modes. BC accounted for 5–20% of total PM mass across all commute modes with an average fraction of ∼7% of PM_2.5.

The study took place from April 2014 to November 2015 and encompassed 161 individual commutes. Since then, the Capitol Corridor Joint Powers Authority has introduced California-built Siemens Charger Clean Diesel-Electric Locomotives on Sacramento’s Capitol Corridor route and begun testing of cleaner-burning renewable diesel fuel. These new technologies reduce particle emissions by about 90%. This transformation is part of a statewide effort led by the California State Transportation Agency (CalSTA) to modernize California’s intercity, commuter, and urban rail systems to improve services, increase safety, and reduce harmful pollutant emissions.

The study has useful implications for our efforts to link transportation and land use planning to develop more sustainable communities. One important finding is the need for more light rail and dedicated bike paths, as well as cleaner locomotives. In addition, the portable technologies we employed to monitor air pollution levels in this project provide us with an important new tool for studying personal exposures and locating air pollution hotspots in disadvantaged communities and elsewhere.

—CARB Research Division Chief Bart Croes

Resources

• Walter Ham, Abhilash Vijayan, Nico Schulte, Jorn D. Herner (2017) “Commuter exposure to PM, BC, and UFP in six common transport microenvironments in Sacramento, California,” Atmospheric Environment doi: 10.1016/j.atmosenv.2017.08.024