Study: Pollution from personal care products comparable to tailpipe emissions in Boulder

Motor vehicles have long been recognized as a dominant source of pollution. But a new NOAA-led study shows that during the morning rush hour in Boulder, Colorado, the trail of chemical vapors emitted by personal care products that commuters use on their skin and hair are comparable in magnitude to the  emissions of major components of vehicle exhaust.

People, it turns out, are a major source of pollution too.

“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.”

This study, published in the journal Environmental Science and Technology, is in line with other recent findings that chemical emissions from personal care products can contribute significantly to urban air pollution.

Smooth, silky and volatile

D5 Siloxane, short for decamethylcyclopentasiloxane, is a common ingredient added to shampoos and lotions and other personal care products to give them a smooth, silky feeling. Siloxane belongs to a class of chemicals called volatile organic compounds or VOCs, which are designed to evaporate. Once released into the atmosphere, 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 like benzene, that 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.

Peaks and patterns

Siloxane and benzene weren’t coming from the same source, but Coggon and his colleagues recognized  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 findings support an emerging body of research into the role of consumer and industrial products as  sources of urban air pollution. A recent study in Science, led by co-author Brian McDonald, another CIRES scientist working at NOAA, found that consumer and industrial products, including personal care products, household cleaners, paints, and pesticides, produced around half of the VOC emissions measured in Los Angeles during an intensive investigation of air pollution sources in 2010.

"Our new study provides further evidence that as transportation emissions of VOCs have declined, other sources of VOCs, including from personal care products, are emerging as important contributors to urban air pollution," McDonald said.

The new study also demonstrates that siloxane is a good indicator of of the presence of emissions from personal care products. “Siloxane is a marker,” Coggon said. “Now we have a very good tracer for understanding the emissions patterns of VOCs emitted from personal care products.”

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.