A resin used for making non-wrinkle shirts releases formaldehyde, which could cause contact dermatitis for some people; no disclosure requirements exist, and in 2008 the U.S. Government Accountability Office tested formaldehyde in clothing and found that generally the highest levels were in non-wrinkle shirts and pants. In 1999, a study of the effect of washing on the formaldehyde levels found that after 6 months after washing, 7 of 27 shirts had levels in excess of 75 ppm, which is a safe limit for direct skin exposure.
How many months of washing did a 1999 study look at?
6