The Environmental Protection Agency (EPA) has not conducted a recent evaluation of demand for Integrated Risk Information System (IRIS) toxicity assessments with input from users inside and outside EPA. The health effects information in IRIS provides fundamental scientific information EPA needs to develop human health risk assessments. Specifically, EPA issued a needs assessment report in 2003, which estimated that 50 new or updated IRIS toxicity assessments were needed each year to meet users' needs. This book determines the extent to which EPA has evaluated demand for IRIS toxicity assessments from users inside and outside EPA; EPA's process for nominating and selecting chemicals for IRIS toxicity assessment and strategies for addressing any unmet agency needs when IRIS toxicity assessments are not available, applicable, or current.