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Introduction
Background
Over 4.9 million barrels of crude oil was released into the Gulf of Mexico between the explosion of the Deepwater Horizon rig on 20 April 2010 and the top-capping of the wellhead on 15 July 2010 (United States Coast Guard 2011). As part of the oil spill response and cleanup (OSRC), approximately 1.8 million gallons (6.8 million liters) of oil dispersant was applied both to the sea surface [1.07 million gallons (4.05 million liters)] and directly into the stream of oil leaving the wellhead 5,000 feet (1.5 km) underwater [0.77 million gallons (2.9 million liters)] (United States Coast Guard 2011). Dispersants are typically used to reduce the interfacial tension between crude oil and water and facilitate the breakup of oil slicks into small droplets that are thought to be more easily dispersed by natural processes such as wind and wave action (Chapman et al. 2007). Two dispersants were used in the Deepwater Horizon spill response: Corexit™ EC9500A (9500A), which was applied at both the water surface and the subsurface wellhead, and Corexit™ EC9527A (9527A), which was applied only at the water surface (Kujawinski et al. 2011). Both dispersants are composed of propylene glycol and organic salts including dioctyl sodium sulfosuccinate (DOSS). Additionally, 9500A contains petroleum distillates, whereas 9527A does not contain petroleum distillates but does contain 2-butoxyethanol (Wise and Wise 2011).
Dispersants were applied to the surface either through aerial spraying or by vessels within 3 nautical miles (5.5 km) of the wellhead area. Aerial application consisted of both 9527A and 9500A from 22 April until 22 May, after which 9500A was used exclusively. Vessels in the wellhead area applied 9500A exclusively (BP Gulf Science Data 2016a). Subsurface application of 9500A was accomplished through a remotely operated underwater vehicle injecting dispersant directly into the stream of oil leaving the wellhead (BP Gulf Science Data 2016b). Based on these uses, the most likely avenues for human exposure among responders are from dermal exposure and from inhalation of dispersant aerosol droplets.
Previous epidemiologic studies have found adverse health effects associated with oil spill cleanup work (Aguilera et al. 2010; Laffon et al. 2016). Effects have included increased lumbar pain, migraine, dermatitis, eye and throat irritation, and respiratory symptoms. Most epidemiologic studies have focused on the acute effects of crude oil exposure during spill cleanup, although Zock...