The Centers for Disease Control study (PDF) sounded reassuring when it was made public in 2007. Hurricane Katrina survivors didn't have to worry about reports that there were harmful levels of formaldehyde in their trailers. The air was safe to breathe and the contamination would not reach a "level of concern" as long as they kept the windows open.
Today, senior CDC officials acknowledge that the study was based on a fundamental error.
An agency standard says that people exposed to as a little as 30 parts of formaldehyde per billion parts of air (ppb) for more than two weeks can suffer constricted airways, headaches and rashes. The trailers all measured above that level.
But the scientists who conducted the study used a much higher agency standard to evaluate the formaldehyde in the trailers: instead of 30 parts per billion, they said health dangers wouldn't occur until the substance reached 300 ppb, 10 times greater than the long-term standard. According to the CDC, people exposed to that amount for just a few hours can suffer respiratory problems and other ailments.
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