threshold levels of exposure to environmental risks are crucial for regulatory agencies to determine safe limits for harmful contaminants. Though any level of exposure to carcinogens is harmful and regulatory agencies use linear dose-response models to estimate the risk to human health, they usually assume threshold levels exist for non-carcinogens. But recently, two scientists have found no apparent threshold for health risks even with non-carcinogens, and, in some cases, higher risks per unit of exposure dose at low levels (Public Library of Science (Medicine), Vol 2, No 12, December, 2005).

Donald T Wigle from the Centre for Population Health Risk Assessment, University of Ottawa, Canada, and Bruce P Lanphear from the Cincinnati Children's Environmental Health Center, Ohio, the us , studied dose-response relationships over exposure ranges far below those generally used in animal studies for four of the most widespread and extensively studied environmental hazards