Oil spill into seawater. Evaporation and human exposure to benzene
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Background: The Norwegian oil spill preparedness focuses on developing response methods that limits the health risk of personnel performing response operations during oil spills at sea. Two types of crude oil, condensates and light crude oils, are in increasing production and transportation on the Norwegian Continental Shelf (NCS), and it has been assumed that these oils may form oil films that are too thin for effective recovery by traditional response techniques. Condensates and light crude oils may have a high content volatile organic compounds (VOCs), such as benzene, that evaporate when oil is spilled at sea. Some of these compounds are associated with adverse chronic health effects such as cancer and effects on the nervous system. Although several previous studies have attempted to measure or model the exposure levels during actual oil spills at sea, knowledge about personal exposure levels in air and biological uptake of volatile compounds is scarce. Also, the effect of using personal protective equipment (PPE) needs further documentation.
Aims: The main objective of this thesis was to gain more knowledge about personal benzene exposure levels during bulk release of fresh crude oil at sea. The objectives of Paper I was to measure the air concentration of VOCs evaporating from a thin oil film formed on the seawater surface by different condensates and light crude oils, and to study how physicochemical properties of the fresh oil affects the air concentration of benzene with time and temperature. The objective of Paper II was to study the personal exposure to benzene during a two-day field study at sea involving several releases of fresh crude oil. In Paper III the objective was to study the association between airborne exposure and biological uptake and the effectiveness of wearing PPE.
Material and methods: A total of nine condensates and light crude oils were included in an experimental bench-scale study. A glass chamber was filled with seawater before fresh oil was applied to form a thin oil film on the seawater surface. Tests were performed with each oil at 2°C and 13°C with equal test conditions. Active automated thermal desorption tubes (ATD-tubes) were used to measure the concentration of specific VOCs in air 10 cm above the oil film in 5 min intervals for a total of 30 min. Continuous air measurements of total VOCs (TVOC) were also performed. Air concentration models were developed for benzene for the first 5, first 15 and last 15 min of sampling to identify determinants having a significant effect on the air concentration of benzene. A total of 22 subjects were recruited during a full-scale field study at sea. Six releases of crude oil were performed over two consecutive days with two different types of fresh crude oil. Personal exposure to benzene was assessed a priori based on the participants work tasks, and three exposure groups (high, low or background) were developed. Continuous air measurements of VOCs were performed in each of the five boats to characterize the overall exposure levels. Full-shift personal exposure measurements were performed with passive ATD-tubes on both days of oil release and urine samples were collected pre- and post-shift to measure biological uptake. All subjects completed a questionnaire before and after their work-shift about their work, smoking habits and use of PPE.
Results: For all oils the highest air concentration of TVOC was measured within 2 min after application of oil, but the concentration rapidly declined to <14 % of the peak concentration within the 30 min of sampling. The TVOC concentration was significantly higher at 13°C than at 2°C during the first 5 and 15 min. The ATD measurements also indicated a rapid decline in the air concentration of benzene, toluene, ethylbenzene and n-hexane, while xylene and naphthalene did not show comparable decline with time. The air concentration models for benzene indicated that content of benzene in fresh oil and oil group (condensate/light crude oil) were significant determinants in the first periods of sampling (first 5 and 15 min). The total variance in the air concentration of benzene explained by these determinants was 63– 67 %, while pour point could explain 73 % of the total variance in the last period of sampling (last 15 min). Although temperature was not a significant determinant in the model, the air concentration was higher at 13°C than at 2°C. In the full-scale field study, the air concentration of TVOC was five times higher during release of light crude oil compared to heavy crude oil. The overall exposure levels were low, characterized by short periods (<1 h) of high exposure associated with release of oil. Subjects in the ‘high exposure’ group, located in the small boats close and downwind from the oil slick, were exposed to the highest air concentrations of benzene (range: 0.013-1.52 ppm), and four subjects exceeded the 12-hr occupational exposure limit (OEL) for benzene of 0.6 ppm. Five subjects who reported not wearing respirators had detectable concentrations (range: 0.5–3.3 μmol/mol) of S-phenylmercapturic acid (SPMA), a biomarker of benzene exposure, in post-shift urine, but the concentration did not exceed the recommended biological exposure index (BEI). Subjects wearing respirators did not have detectable concentrations of post-shift urinary SPMA, even when exposure levels exceeded the OEL.
Conclusion: Benzene evaporates rapidly from a thin oil film of condensate or light crude oil at both 2°C and 13°C, even when the content of benzene is the fresh oil is relatively high. However, evaporation appears to be slower for oils with a high pour point compared to oils with a low pour point. Personnel located close and downwind from an oil slick during the initial stages of a bulk spill of fresh light crude oil at sea may be exposed to benzene levels exceeding the OEL. Although biological uptake of benzene is possible, use of appropriate respirators prevents uptake at these exposure levels. The risk of exposure is mainly associated with the content of benzene in the fresh oil, but a high content of wax may prolong the time of exposure.