In a study which could impact air quality regulation, researchers said Wednesday that chronic exposure to one of the major components of smog significantly raises the risk of dying from lung disease.
The study found that the risk of dying from respiratory disease increased as much as 50 percent as a result of long-term exposure to high concentrations of ground-level ozone.
Previous studies have already linked spikes in ground level ozone levels to heart attacks and severe asthma attacks and shown that long-term exposure to the tiny particles of soot and dust found in smog is a risk factor for heart and lung disease.
This is the first study to look at the long-term health impacts of ozone, which is formed through the chemical reaction between sunlight and the nitrogen oxides spewed from vehicle exhaust and industrial emissions.
"Many studies have shown that a high-ozone day leads to an increase in risk of acute health effects the next day, for example, asthma attacks and heart attacks," said study co-author George Thurston of New York University's medical school.
"What this study says is that to protect the public's health, we can't just reduce the peaks, we must also reduce long-term, cumulative exposure."
The researchers estimate that the risk of dying from respiratory causes rises four percent for every 10 parts-per-billion increase in exposure to ozone.
"World Health Organization data indicate that about 240,000 people die each year from respiratory causes in the United States," said lead author Michael Jerrett of the University of California at Berkeley.
"Even a four percent increase can translate into thousands of excess deaths each year. Globally, some 7.7 million people die from respiratory causes, so worldwide the impact of ozone pollution could be very large."
The Environmental Protection Agency will be reviewing its ozone standards in the coming year, which currently do not protect against the long-term cumulative effects of ozone exposures but instead set the standard for short-term exposure at 75 parts per billion.
The study found that even in a city like New York where ozone levels are almost never that high, the risk of death from respiratory disease was increased by 25 percent due to long-term exposure.
The study published in the New England Journal of Medicine analyzed data on some 450,000 people who were followed from 1982 to 2000 as part of an American Cancer Society study and compared it to ozone data collected in 96 US cities between 1977 and 2000.