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A quarter of Angelenos breathe noxious freeway pollutants every morning

By Alison Hewitt
Originally published in UCLA Newsroom

Although air quality has improved dramatically in Los Angeles in recent decades, a joint study by UCLA and the California Air Resources Board suggests that roughly a quarter of Angelenos are exposed to noxious plumes of freeway fumes almost every morning — far more people than previously believed.

Researchers found that overnight atmospheric conditions concentrate freeway pollutants in a plume stretching 1.5 kilometers (approximately 0.93 mile) downwind, seeping inside homes and buildings, and lingering as late as 10 a.m. The same effect would be expected downwind of any highway nationwide, the researchers said.

Half of the residents of the greater Los Angeles area live within these impact zones around freeways, meaning that about a quarter are on the downwind side of a freeway on any given day.

The 1.5 kilometer measurement is in striking contrast to earlier studies in the United States and Australia showing that daytime pollutant concentrations extended no more than about 300 meters (about 0.19 mile) downwind of major roadways, and confirms an earlier UCLA study that showed the same result at a single coastal location.

Professor Suzanne Paulson of UCLA’s Department of Atmospheric and Oceanic Sciences and the Institute of the Environment and Sustainability headed the study, working with Professor Emeritus Arthur Winer of UCLA’s Fielding School of Public Health and led by Wonsik Choi, a postgraduate researcher in Paulson’s lab. The findings were published in December in the journal Atmospheric Environment.

The researchers measured pollutant concentrations upwind and downwind of the 91, 210, 110 and 101 freeways using a zero-emission vehicle equipped with specialized instruments to quantify the amount of ultrafine particles and other tailpipe pollutants such as nitrogen oxide and carbon monoxide. Starting before morning rush hour, a researcher drove back and forth several times on surface streets perpendicular to the four freeways, visiting each freeway five times.

“This is happening around every freeway,” Paulson said. “It’s clear that heavily trafficked roadways have a large impact on downwind populations, and a similar situation likely happens around the world in the early morning hours. The particles tend to end up indoors, so a lot of people are being exposed inside their homes and schools.”

Although closed windows help block the particles from seeping inside, previous studies have shown that indoor pollution levels still reach 50 to 70 percent of outdoor levels, the researchers noted.

By knowing which way the wind is blowing, the researchers can extrapolate their findings to any freeway. Their latest measurements, and the most common nighttime wind patterns, result in the following impact zones:    

Sides of freeways in greater Los Angeles with concentrated pollution in early morning

Location  Freeway   Typically polluted side  How determined
Carson 110 Mostly east, sometimes west Current study
Claremont 210 South Current study
Downtown L.A.  101 South Current study
Paramount 91 South Current study
San Fernando Valley 101 North Extrapolated from wind data
San Fernando Valley 118 South Extrapolated from wind data
San Fernando Valley 405 Varies; West and east Extrapolated from wind data
Santa Monica 10 South Earlier study
West L.A. 405 West Extrapolated from wind data

The morning concentration of pollutants near highways is due to a combination of the nighttime cooling of the atmosphere, called a nocturnal surface inversion, and the weak evening breeze. At night, cool air sinks, trapping polluted air close to the ground, where it can’t interact with cleaner air from above. As drivers create more emissions overnight and during morning rush hour, Los Angeles’ mild “land breeze” pushes the increasing pollution in a plume to one side of the freeway. The cold layer traps the plume until well into the morning rush hour, when sun-warmed air begins to rise and a stronger sea breeze takes over, mixing pollutants throughout the atmosphere and diluting their influence.

“While freeway plumes vary slightly from location to location, all of the sites exhibited highly elevated traffic-related pollution at distances of at least 1.5 kilometers on the downwind side of the freeway in the early morning,” Choi said.

Freeway pollutants have been linked to increases in asthma, heart attacks, strokes, diabetes, low birth weight, pre-term births and other ailments. So what can people do to limit their exposure to polluted air?

“If your home is within about 1.5 kilometers of a freeway, you may want to close your windows during the early morning hours,” Winer said. “It’s also best not to run or otherwise heavily exercise within the 1.5 kilometer impact zone in the early morning.”

Likewise, schools near freeways should avoid holding gym classes first period, and people with breathing difficulties should filter their indoor air, the researchers suggested.