Although chlorine and most of its derivates are known toxic agents, it has been pronounced as a safe disinfectant for water treatments. More detailed analyses and extended studies concerning chlorine safety have only started recently. The objective of this article was to review data on the use of chlorine in pool environments, the resulting chlorination by-products in these environments and their potential effects on allergic and respiratory health in humans. The MEDLINE database search comprised articles from 1966 to August 2006. Additional studies were identified by searching references of already published articles. A total of twenty-one studies evaluating effects of chlorine and its byproducts on allergic or respiratory health were included in the analysis. Exposure to chlorination by-products through swimming pool attendance showed adverse health effects on children, subjects occupationally exposed, athletic swimmers and asthmatic subjects. These adverse effects were seen despite the presence of official directives in most countries to control and regulate the use of chlorine for water disinfection. Contact to chlorination by-products might not be the leading reason for poor respiratory health, but might not be as harmless as earlier thought. In particular, baby swimming in chlorinated pools is highly questionable.