Smog caused by ground-level ozone isn't just an outdoor air problem. A new study shows that when the irritant's level rises outside, the number of people inside suffering from so-called "sick building syndrome" also increases. (Ozone, an air-polluting oxygen molecule (O3), forms when sunlight strikes motor vehicle tailpipe emissions.)

"We found that outdoor air pollution, ozone, is associated with symptoms of lower-respiratory and upper-respiratory stress that occur in buildings to workers," says environmental health scientist Michael Apte of Lawrence Berkeley National Laboratory in California, who analyzed U.S. Environmental Protection Agency (EPA) data gathered on office air quality across the country. "These symptoms are prevalent at fairly high levels throughout the U.S. and are similar in other parts of the world."

Sick building syndrome is a term used to describe a broad range of ailments, including dry eye, congestion, difficulty breathing, fatigue and headaches that strike workers inside office buildings but disappear when they leave the premises. Previous studies have shown that individuals may exhibit such symptoms when irritating chemicals and particles are found in the air and that ventilation with clean air can help alleviate or prevent the problem.

The EPA from 1994 to 1998 tested indoor air quality and surveyed office workers in 100 buildings in 37 cities—from the smoggiest to the cleanest—across the country. Nearly all of the buildings had mechanical ventilation systems rather than windows or other natural means of circulating air. Officials measured air quality across a broad range of seasonal conditions, from an office in North Dakota on a –18-degree Fahrenheit (–28-degree Celsius) day to a workplace in Arizona on a 108-degree F (42-degree C) day.

The EPA found that, on average, even in buildings with no special history of sickness, nearly 19 percent of workers surveyed complained of dry eye, 21 percent felt congested on the job, 4 percent complained of difficulty breathing, more than 19 percent felt fatigued, and more than 15 percent reported having headaches while at work.

Apte and his colleagues compared the EPA office data with the agency's measures of local atmospheric ozone levels to determine whether they were linked. Their findings, published in the journal Indoor Air: the number of workers suffering symptoms indoors increased with the amount of smog outside—even at levels below the national limit of 80 parts per billion of ozone in air over an eight-hour period set by the EPA.

"Our body is telling us that there is something irritating in the air," Apte says. "People in the work condition are under all kinds of stress, but it does seem to be a real physical response."

The researchers found that the type of air filter used in a building's ventilation system was also tied to the number of ill people. "There is a six times greater likelihood that these symptoms will occur if you have both higher ozone levels and the polyester or synthetic filters," Apte says, "than if you are in lower ozone levels and using a fiberglass filter."

No specific cause for sick building syndrome has been identified, but Apte speculates that the symptoms are due to unstable ozone molecules chemically interacting with the wide range of materials found in an office building, beginning with the polyester air filter. "Glass is really a very inert material," Apte says of the fiberglass filters. "On the other hand, polyester is a polymer and it's got a lot of bonds in there that are capable of being broken up by ozone."

Ozone also reacts with a slew of plastics and other reactive surface molecules, ranging from carpet fibers to the skin's natural oil, to produce toxic chemicals like formaldehyde and other irritants. "Reaction chemistry suggests that the stuff being created is worse than ozone itself," Apte says. "It's like atmospheric chemistry but it's going on in building atmospheres rather than out in the sunlight." In other words, ozone seeping into buildings combines with other chemicals to produce more noxious air.

Researchers estimate that exposure to unhealthy ozone is cumulatively greater from such indoor air pollution than from concentrations that are as much as 10 times higher outside, because people spend the majority of their day indoors. As a result, sick building syndrome costs the U.S. economy as much as $60 billion annually in lost productivity. Installing filters designed to catch ozone before it gets into office air—as well as cutting back on the ozone-forming tailpipe pollution of all those commuters—would help, Apte says. And the problem could be reduced by 75 percent, the study finds, simply by switching from polyester to fiberglass filters in the smoggiest areas.

After all, the ventilation system turned out to be the culprit in the first sick building affliction: Legionnaire's disease, named for a mysterious pneumonia outbreak at an American Legion convention in 1976. The illness was found to come from Legionella bacteria thriving in the hotel's cooling tower and spread through its air conditioning system.

Outdoor ozone air pollution interacting with indoor filters and environments may prove a root cause of at least some of the otherwise unexplained sick building symptoms, Apte says. "I'm hoping that this ozone finding continues to provide an explanation of at least some of the variance in the distribution of symptoms people experience in buildings, though I don't believe this can explain all of them," he notes. "Some day it will no longer be called sick building syndrome."