August 26, 2019
Motor vehicles have long been recognized as a dominant source of pollution. But a new study led by the National Oceanic and Atmospheric Administration (NOAA) shows that—during the morning rush hour in Boulder, Colorado—the trail of chemical vapors emitted by commuters who have applied a variety of personal care products to their skin and hair is comparable in magnitude to the emissions of major components of vehicle exhaust.
“We detected a pattern of emissions that coincides with human activity,” said lead author Matthew Coggon, a CIRES scientist working at NOAA’s Earth System Research Laboratory. “People apply these products in the morning, leave their homes, and drive to work or school. We see emissions spike in the morning and show a lower peak again at the end of the day.”
The study findings, published in the journal, Environmental Science and Technology, confirms other recent findings, which demonstrate that chemical emissions from personal care products can contribute significantly to urban air pollution.
Among the chief culprits: D5 Siloxane, short for decamethylcyclopentasiloxane, a common ingredient added to shampoos, lotions, and other personal care products to give them a smooth, silky feeling.
Siloxane belongs to a class of chemicals called volatile organic compounds (VOCs), which are designed to evaporate. Once released into the atmosphere, NOAA says, sunlight can catalyze reactions between VOCs, nitrogen oxides, and other compounds to form ozone and particulate matter—two types of pollution that are regulated because of their effects on air quality and human health.
Coggon and his colleagues measured VOCs from the roof of NOAA’s Earth System Research Laboratory in December 2015 and January 2017—and from a mobile laboratory driving around Boulder during rush hour in February, 2016. They sampled everything they could, including compounds such as benzene, which are known markers of vehicle exhaust.
“We were surveying the air, monitoring every chemical compound our instrument was sensitive to—about 150 compounds,” said Coggon. From that soup of chemicals, one compound in particular caught their attention. “We found a big peak in the data but we didn’t know what it was,” he said.
Based on the measurements, Patrick Veres, a NOAA scientist and co-author on the paper, suggested the suspect might be a chemical known as D5 siloxane, a refined petroleum product he recognized from another research project. When Coggon’s team saw that siloxane levels appeared to rise and fall throughout the day in step with measurements of benzene emissions from traffic, they initially theorized siloxane was a component of vehicle exhaust. But when they tested tailpipe emissions directly and took roadside measurement, siloxane was absent.
Siloxane and benzene weren’t coming from the same source, but Coggon and his colleagues recognized that the chemicals were linked with a particular human behavior: Commuting.
By studying their data hour-by-hour, they realized siloxane emissions peaked in the morning, when people put on personal care products and went outside into their cars or buses. That’s when benzene emissions went up too. Emissions of both chemicals decreased during the day, then peaked again during the evening commute. The evening peak of siloxane emissions was lower than in the morning, because, they theorized, the personal care products had largely evaporated throughout the day.
The research team is looking at other chemicals in personal care products that may also spike in the morning, as people commute. “We all have a personal plume, from our cars and our personal care products,” said Coggon.
Research contact: @NOAAResearch