Joey,
Might have been due to the arsenic used to be in treated wood.
Arsenic is a notoriously deadly poison, but for twenty years it was the most common preservative applied to wood used to build playgrounds and outdoor decks in neighborhoods across the United States. As a result, these structures, where children and families play and eat, are the largest source of arsenic exposure for an overwhelming majority of Americans.
http://www.healthybuilding.net/arsenic/index.html