Table 3.

Description of study time periods.

Time period	Dates	Major activities
1a	22 April to 14 May 2010	Oil was being released at the wellhead near the floor of the Gulf. Efforts to stop the release by containment were unsuccessful. Drilling started on a relief well. Aerial application of dispersant COREXIT™ 9527A (9527A) and 9500A (9500A) to the water surface began across the Gulf. In situ burns started. Oil reached the LA and MS shore, and beach cleanup started.
1b	15 May to 15 July 2010	Drilling began on a second relief well. Only 9500A dispersant continued to be applied by plane. 9500A dispersant was injected at the wellhead to break up the oil and applied by ship to the surface of the water near the wellhead to reduce THC air concentrations. In situ burning continued. Offshore skimming of oil and flaring by the rig vessels started. Beach cleanup continued, but now included AL and FL, and onshore and offshore decontamination of vessels and equipment started. On 15 July, the well was mechanically capped with a functional BOP and the oil flow stopped, but the well cap and well casing were still under considerable pressure from the oil formation. Flaring of oil and gas ceased with the well capping on 15 July.
2	16 July to 10 August 2010	The well was “static killed” (relieved pressure on well casing by pumping drilling mud and other components into the well) on 10 August. In situ burning ended by 19 July. Offshore skimming ended in early August but nearshore skimming continued. Beach cleanup and decontamination of vessels and equipment continued. The cleaning of the jetties and other structures such as bridges began.
3	11 August to 30 September 2010	The first relief well intersected the original well ~18 000 feet below the Gulf floor on 16 September. Heavy mud and cement were delivered at the base of the well to close off the well. The well was permanently sealed on 19 September. Underwater equipment used in the response was removed or repositioned. Large-scale decontamination of the vessels started. Nearshore skimming continued on a limited basis. Beach cleanup continued but started to decline. Efforts to clean the marshes began during this time period but only affected LA and MS.
4	1 October to 31 December 2010	Decontamination of the vessels and equipment continued. By 31 December, essentially all vessels had been decontaminated and were considered out of service. Beach and marsh cleanup continued, along with the cleaning of rock jetties, by an increasingly smaller number of people. Nearshore skimming ended in October. The decontamination and repair of boom was completed by the end of this time period.
5	1 January to 31 March 2011	Beach cleanup continued by an increasingly smaller number of people.
6	1 April to 30 June 2011	Beach cleanup continued by an increasingly smaller number of people. Distinguished from time period 5 because of the warmer ambient air temperatures that resulted in higher THC exposure levels.