No “Safe” Threshold for Ozone?

Increased ground-level ozone in the United States is responsible for thousands of premature deaths annually, according to a new study by Bloomberg School researchers—news that means the Environmental Protection Agency may need to revise its ozone pollution standards… again.

The study, reported in the November 17, 2004, issue of the Journal of the American Medical Association, is one of the largest ozone pollution studies ever conducted. It looked at 95 large urban centers, containing 40 percent of the total U.S. population, during the period from 1987 to 2000. The study provides strong evidence that when ozone levels go up, there are increased numbers of adult non-accidental deaths for several days afterward.

“This is an important finding, because it measures ozone levels as they are affecting health now,” says Epidemiology chair Jonathan Samet, MD, one of the co-authors. Evidence linking high levels of particulates in the air to increased mortality led the EPA to propose tougher pollution standards in 1997 that were eventually challenged (and upheld) in the U.S. Supreme Court. Part of those revised standards also reduced permissible ozone levels, based upon studies showing decreased lung function at levels then commonly measured in the U.S.

The study suggests that if there is a “threshold limit” at which levels of ozone present no risk to human health, the U.S. has not yet reached it. “Even as we lower levels of ozone we continue to see health effects,” says Samet, the Jacob I and Irene B. Fabrikant Professor of Health, Risk and Society. “There needs to be an adequate margin of safety. This study indicates we still have a way to go.”

Ground-level ozone is created by chemical reactions between oxides of nitrogen and volatile organic compounds in the presence of strong sunlight. That is why high levels of ozone are more common in summer months. Emissions from industrial facilities and electric utilities, motor vehicle exhaust, gasoline vapors and chemical solvents are major sources of ozone-creating pollutants, according to the EPA.

“This is the first piece of solid evidence linking ozone to increased deaths,” says co-author Francesca Dominici, PhD, associate professor of Biostatistics. Dominici and her colleagues employed a sophisticated regression model to factor out other causes such as heat waves, the flu season and increased levels of particulate matter in the air—in order to devise a way to compare mortality rates in populations where ozone levels changed but not much else did.

It proved to be a challenging problem of sorting. Dominici explains: “We know, for instance, that mortality is higher during heat waves, and that is often when ozone levels peak.” Since it is difficult to determine the cause of excess deaths during these periods, the researchers decided to leave them out entirely, excluding all days in which temperatures exceeded 80 degrees Fahrenheit. The result, says Dominici, is “a very conservative” estimate of 3,767 excess deaths in the 95 studied urban areas each year for every 10 parts per billion increase in daily ozone levels. Surprisingly, the numbers indicate a substantial health burden in all age groups, not just the elderly. A slightly higher effect was found for individuals with respiratory or cardiovascular disease. Dominici and colleagues note that it is too early to even theorize what mechanism is causing the excess deaths. 

In addition to Dominici and Samet, Biostatistics assistant scientist Aidan McDermott, PhD, and Biostatistics chair Scott Zeger, PhD, were co-authors. Research scientist Michelle Bell (now at Yale University) was lead author.

“Although we received a lot of attention from the press, there was not much industry reaction to these findings,” Dominici says. “But that usually comes later, when new regulations are proposed. Now we need to begin to assess what the public health effect of new regulations might be.”