OBJECTIVE Acute chemical incidents (i.e., uncontrolled or illegal release or threatened release of hazardous substances lasting <72 hours) represent a substantial threat to the environment, public health and safety, and community well-being. Providing a timely and appropriate public health response can prevent or reduce the impact of these incidents. METHODS 1999-2008. METHODS The Hazardous Substances Emergency Events Surveillance (HSEES) system was operated by the Agency for Toxic Substances and Disease Registry (ATSDR) during January 1991-September 2009 to describe the public health consequences of chemical releases and to develop activities aimed at reducing the harm. This report summarizes types, frequency, and trends in public health actions taken in response to hazardous substance incidents in the nine states (Colorado, Iowa, Minnesota, New York, North Carolina, Oregon, Texas, Washington, and Wisconsin) that participated in HSEES during its last 10 full years of data collection (1999-2008). RESULTS Of the 57,975 HSEES incidents that occurred during 1999-2008, a total of 15,203 (26.2%) incidents resulted in at least one public health action taken to protect public health. Evacuations were ordered in 4,281 (7.4%) HSEES incidents, shelter in place was ordered in 509 (0.9%) incidents, and access to the affected area was restricted in 10,345 (25.9%) incidents. Decontamination occurred in 2,171 (3.7%) incidents; 13,461 persons were decontaminated, including 1,152 injured persons. Actions to protect public health (e.g., environmental sampling or issuance of health advisories) were taken in 6,693 (11.5%) incidents. The highest number of evacuations and orders to shelter in place occurred in Washington (n = 558 [16.1%] and n = 121 [3.2%], respectively). Carbon monoxide and ammonia releases resulted in the highest percentage of orders to evacuate and shelter in place. The most frequently reported responders to chemical incidents were company response teams. CONCLUSIONS The most frequent public health response was restricting access to the area (26% of incidents), public health actions (12%), evacuation (7%), decontamination (4%), and shelter-in-place (1%). Ammonia and carbon monoxide were associated with adverse health effects in the population and the most public health response actions. Therefore, these chemicals can be considered a high priority for prevention and response efforts. PUBLIC HEALTH IMPLICATIONS: States and communities can collaborate with facilities to use the information collected through community right-to-know legislation and this report to improve chemical safety and protect public health and the environment, such as being prepared to handle the most common chemicals in their area and probable public health actions.