Many older wooden playgrounds, swing sets and outdoor furniture were made of processed or treated wood impregnated with chromated copper arsenate (CCA). Arsenic, a chemical typically found in this product (and a known human carcinogen) can be absorbed through the skin or accidentally ingested when children put their hands in their mouths. The soil around play equipment may be contaminated as well. If the old equipment cannot be removed or replaced, the wood should be sealed with several coats of paint or oil-based sealant to stop the chemicals from leaching. More info at the Consumer Product Safety Comission.