Unfriendly skies: flying may be hazardous to your health--in more ways than one. (Your Health).

Long before cockpit doors were reinforced by federal order and United Airlines handed its pilots stun guns to zap would-be terrorists, flying carried its own health risks. For the moment, let's forget about the millions of pounds of emissions planes create each year, which contribute to smog and global warming. Let's ignore their trail of noise pollution, which makes it difficult to escape civilization even in remote wilderness.

For now, we can also overlook some of the most obvious ways flying can beat you up--jet lag, fake food, changes in air pressure (which can cause long-term damage through "popped" ear syndrome), dehydration (high-altitude air has a humidity close to zero), aggravation from delays, increased risk of heart attack, cramped conditions (which on rare occasions lead to fatal blood clots) and motion sickness. Instead, let's focus on the less noted, but serious threats that come from breathing stale air and being exposed to pesticides and radiation.

Beware of Breathing

The first clear indication that the air (or lack thereof) in an airplane could be unhealthy was noticed in a 1977 Homer to Kodiak, Alaska flight. The flight was delayed for four and a half hours, giving a chance for the flu of one coughing woman to spread to 38 of the 54 passengers. "People get sick from flying all the time," says indoor-air expert Harriet Burge, a University of Michigan scientist. "But usually it's impossible to prove. This happened to be a perfect case. You had one Kodiak doctor who asked himself why all these tourists got influenza. If the plane had landed in Chicago, each person would have seen a different doctor. No one would have known why they got ill."

And that Kodiak case happened during the good old days: Just a couple years later, in 1979 and 1980, when fuel costs skyrocketed, airlines realized they could save money by giving passengers less fresh air. On older planes, totally fresh air was circulated every three minutes; after the fuel crisis, outside air was mixed with half recirculated air about every six or seven minutes. In the early 1990s, Boeing 767s and Airbus A-300s dropped the fresh air part to less than half. As medical writer Michael Castleman put it, "This gives that sneeze from seat 3C more of a chance to reach 43F."

Low levels of oxygen may explain why you feel like hell after a flight. As Health magazine explains, "Outdoor air contains about 400 parts per million of carbon dioxide. Levels of more than 800 typically set office workers grousing. The average reading in airplanes is 1,756." "Great," you may be thinking, "just what we need are oxygen-deprived pilots." Not to worry; the pilots' cockpit receives 20 times as much fresh air as the passenger section. While no one would argue with treating the pilots well, that still leaves everyone behind the control room rather vulnerable.

The Spray-Happy Skies

Stale air humidified by your fellow passengers' sweat and breath may give you a flu or sinus infection, but unfortunately germ-laden oxygen isn't all you breathe in as you wing across the sky. As Mother Jones reported, "Several major airlines, including American, Continental, Delta, TWA, and US Airways, confirm that they use pesticides `regularly' or `occasionally,' but would not disclose what types they use. Northwest confirms that it sprays residuals--or long-lasting-pesticides on its domestic fleet while the planes are in the hangar."

On some foreign flights, airlines are required by native law to spray while passengers are on board (fortunately, due to complaints, only four countries continue to do this). Although spraying while passengers are present is particularly harmful, even spraying empty planes is a health threat, especially given the poor onboard air circulation.

The National Institute of Environmental Health Sciences and other research groups have linked pesticides with an assortment of ailments including skin allergies, damage to the nervous system, respiratory problems, endocrine disorders, increased chemical sensitivity and cancer. But no federal agency requires spraying. Mother Jones concludes that the airlines do it for cosmetic reasons (heaven forbid someone sees a mouse), but like other forms of pest control there are safer methods available.

A Radiation Shield?

If you're considering packing a gas mask for your next flight, better throw in a radiation shield too. When we're Earthbound, the atmosphere protects us from much of the radiation streaming in from outside the solar system. The higher we go, however, the less protection there is. While zooming about at 40,000 feet we're exposed to 200 times more cosmic radiation than at ground level. After five hours, you absorb roughly as much radiation as you'd get from a chest X-ray. Most government officials pooh-pooh that amount of radiation for occasional fliers as no big deal.

The wild card, however, which no one in the airline industry or Federal Aviation Administration (FAA) likes to talk about, is solar storms--times when the sun releases very large bursts of radiation. "Solar storms can vastly increase the radiation dose you get," says Robert Barish, a radiotherapy physicist and author of The Invisible Passenger: Radiation Risks for People Who Fly. "They're unpredictable. But when they happen you can get as much radiation on a single flight as you normally would on a hundred flights. If you're pregnant and flying to Tokyo on one of those days, you could exceed the maximum recommended dose to the fetus, which increases the risk of birth defects or genetic damage." Fortunately, solar storms happen only occasionally. Still, Barish says the FAA should require airlines to install radiation monitors on planes. The FAA contends it's impractical and unnecessary. Barish says they're acting irresponsibly.

Solar storms or not, many medical authorities, including Dr. Andrew Weil, don't take any radiation exposure lightly: "There is no such thing as a safe dose of radiation. Your risk of genetic and immune system damage correlates with the total amount of radiation you have received over your lifetime, and any amount, however small, adds to that cumulative total and risk." Even government officials concede that normal flight exposure could be a concern for pregnant women, frequent fliers and flight crews.

Rob Boyd, a Boeing 767 captain for Airborne Express and president of Teamsters Union local 1224, says noise levels, air quality and high-altitude radiation exposure remain big concerns for pilots. "The problem is that the FAA and the Occupational Safety and Health Administration (OSHA) are in a turf war over who's responsible, so we have no regulation and no protection."

According to Diana Fairechild, passenger advocate and former flight attendant, airline crews receive higher radiation doses per year than nuclear power plant workers. On her website, Fairechild cites a 1995 Finnish study that found: "1. Flight attendants are two times more likely to contract breast cancer than women who do not fly regularly; 2. Flight attendants suffer a significant increased incidence of bone cancer." Other studies, however, have shown no increased risk. CONTACT: FAA, [phone], www. faa.gov; Diana Fairechild, [phone], www.flyana.com.

MARSHALL GLICKMAN is the author of The Mindful Money Guide: Creating Harmony Between Your Finances and Your Values.