Los Angeles has again topped a list of the cities with the worst smog in the nation, violating federal health standards for ozone an average of 122 days a year.

The annual air pollution rankings, being released Wednesday by the American Lung Assn., were dominated by the Los Angeles Basin and California’s Central Valley, which despite vast improvements over the last few decades still have the nation’s highest levels of ozone and fine particle pollution.

“Air pollution is not just a nuisance or the haze we see on the horizon; it’s literally putting our health in danger,” said Bonnie Holmes-Gen, senior policy director of the American Lung Assn. in California. “We’ve come a long way, but the status quo is not acceptable.”

The report evaluated metropolitan areas based on recorded levels of ozone, the main ingredient in smog, and conducted a separate analysis of fine particles — or soot — the microscopic pollutants that tend to build up in colder, winter months.


The Los Angeles region ranked fourth among metropolitan areas nationwide for short-term spikes in fine particle pollution, coming in behind Fresno, Visalia and Bakersfield. The L.A. Basin tied for third with Bakersfield for annual fine particle concentrations.

The nonprofit advocacy group’s “State of the Air” report derives its rankings from publicly reported measurements of ozone and fine particle pollution from official monitoring sites. Analysts used data from 2010, 2011 and 2012 and averaged the number of bad air days. They also assigned A through F grades to counties.

The nation’s air is far cleaner than decades ago because of stricter emissions standards for coal-fired power plants, vehicles and diesel engines, the report notes. Even as population grows, emissions of the most widespread air pollutants continue to drop.

Greater Los Angeles has reduced ozone levels by more than one-third in the last 15 years, the Lung Assn. says, while fine particle pollution has dropped by half over the same period.


Still, over 147 million people — about 47% of the nation — live in counties with unhealthy air, according to the report. Nearly 30 million of them are in California, where 77% of the population lives in counties with failing grades.

Climate change could make progress on air quality more difficult as an increasing number of hot, sunny days favor the formation of ozone, the Lung Assn. says.

Ozone is a gas generated when pollution from vehicle tailpipes, power plants and factories bakes in sunlight. Ozone inflames the lungs, aggravates asthma and other respiratory illnesses and is especially harmful to children. Fine particulate matter from diesel exhaust, wood smoke and other emissions lodges deep in the lungs and is tied to cancer, heart and lung disease and many other health problems.

The assessment comes as the U.S. Environmental Protection Agency is considering a stricter nationwide smog standard as part of a review required every five years under the Clean Air Act. The current standard, set in 2008 by the George W. Bush administration, limits permissible ozone concentrations to 75 parts per billion over an 8-hour period.


In 2011, President Obama went against the recommendations of the EPA and a panel of scientific advisers to reject a proposal to lower the standard to between 60 and 70 parts per billion. By this summer a scientific panel is expected to again recommend tightening the limit to provide greater public health protections.

Holmes-Gen said the Lung Assn. and environmental groups sued the Obama administration to force it to move forward with new smog limits and urged the EPA to “strengthen this important public health standard.”

tony.barboza@latimes.com

Twitter: @tony.barboza