The EPA (Environmental Protection Agency) has reported that indoor
air contains 2-5 times more pollutants than outdoor air.
The Massachusetts Department of Public Health labels tennis balls as a
source of indoor air pollutants in their August 2006 report “Indoor Air Quality
Testing - Pediatric Asthma Surveillance in Massachusetts Public Schools”.
Tennis balls have a hollow core and some varieties of tennis balls are filled with
nitrogen gas in the center. Nitrogen gas will be released when punctured to allow the tennis ball to be retrofitted to school chair legs exposing maintenance personnel to this gas.
The American Lung Association has recently made statements that they are concerned about indoor air quality.
This is from a report on indoor air quality from the State of Massachusetts –