Exposure Assessment of Ambient Sulfur Dioxide Downwind of an Oil Refinery in Curaçao

Primary sources of atmospheric sulfur dioxide are anthropogenic activities associated with the burning of fossil fuels and industrial processes that may have associations with various morbidities and mortalities. As such, several regional and global regulatory agencies have recommended ambient air limits to reduce environmental exposures. The objectives of this study were to evaluate ambient levels of sulfur dioxide surrounding an oil refinery on the island of Curaçao. Levels which were then compared to literature values and recommended public health guidelines to determine potential public health risks. Concentrations of sulfur dioxide in Curacao were found to be among the highest globally with an increasing trend over time. While the epidemiological evidence does not conclusively suggest there is a health risk from these levels of exposure, SO2 concentrations greatly exceed regulatory and guidance levels and suggest that further emissions control is warranted. Future recommendations include a more rigorous epidemiological study in Curaçao and expand the air monitoring efforts to include areas upwind of the refinery as well as additional petrochemical emissions.

Atmospheric SO 2 is primarily the result of anthropogenic activities associated with the burning of fossil fuels and industrial processes (i.e., oil refineries, coal burning, and biomass combustion) but can also be released naturally from volcanic activities, sea-salt emissions and sulfur gas oxidation [2] [3] [4].Sulfur dioxide (SO 2 ) can be formed from the petroleum refining process and smelting industries, accounting for 15% -25% of the estimated 140 -350 million tons of sulfur compounds (i.e., sulfur dioxide, sulfuric acids and sulfate) released into the atmosphere annually on a global basis [2] [3] [5].
The Wider Caribbean Region (WCR) includes a number of developing countries for which agriculture, oil and gas exploration in conjunction with processing, provide valuable sources of income.As the WCR is one of the most tourism-dependent regions of the world, factors that affect environmental health and sustainability will have inevitable impacts to the economies and quality of life in many already-needy countries.Approximately 100 oil refineries are located within the WCR, with one of largest and oldest refineries, Isla Refineriá, being located in Willemstad, Curaçao [6].Communities downwind of Isla Refineriá and the major thoroughfare (Schottegatweg Ring) circling the refinery and the bay have self reported from questionnaires higher than average frequencies of headaches, nausea, chronic lung ailments, asthma and cancer [7].These self-reported questionnaire results have not been validated.A legacy of human health and environmental issues is the basis of a historical debate and conflict between the public and the local government of Curaçao.However, to our knowledge there has never been an exposure assessment conducted to evaluate ambient atmospheric levels of refinery emissions and their associated public health risks in Curaçao.Sanhueza et al. [8] determined that all areas downwind (≥5 km) of the Isla Refineriá in Curaçao were subject to sulfate contamination exceeding levels which have been previously associated with morbidity (≥8 -12 µg/m 3 ).In addition, the 2007 yearly average SO 2 levels (152 µg/m 3 ) measured downwind of the refinery were double the air quality standards for Curaçao (80 µg/m 3 ) and exceeded acceptable international guidelines (40 -60 µg/m 3 ) by almost 2.5 times [9] [10].In 2009, a court order required the refinery to reduce the excessive SO 2 emissions and particulates starting from January 1, 2010.
Sulfur dioxide has been associated with various morbidities demonstrated by controlled exposure experiments conducted in the early 1950's [11] [12].Epidemiological studies have reported that some sulfur dioxide exposures may be associated with respiratory irritation, bronchoconstriction and have the potential for causing respiratory and pulmonary changes and cardiovascular abnormalities in both healthy and asthmatic individuals [12]   The short term effects from sulfur dioxide exposure at much lower and more plausible episodic concentrations between 0.20 and 1 ppm (524 -2620 µg/m 3 ) were also evaluated using controlled chamber experiments in normal, atopic and asthmatic volunteers [16] [17].Normal and atopic volunteers showed little response at these levels, whereas some atopic volunteers and most of the asthmatics developed bronchoconstriction and respiratory symptoms.Even at elevated ventilation during exercise, there has been limited evidence of SO

Data Analysis
All SO 2 data is expressed in µg/m 3 .All data analysis was performed using Statistica Version 6.1 (Stat Soft, Inc., Tulsa, OK).If data did not meet the assumptions of normality, nonparametric hypothesis tests were performed.If a potential explanatory variable was categorical (e.g., year), the nonparametric Kruskal-Wallis ANOVA was run using α = 0.05.If the Kruskal-Wallis ANOVA was found to be significant multiple comparisons revealed significant differences between categorical factors.Concentrations of SO 2 were compared daily and annually and correlations were also evaluated against environmental factors (i.e., temperature, precipitation, humidity and wind speed).

SO2 Compliance with Public Health Guidelines
The maximum annual mean concentrations for SO 2 that are currently recommended by Curaçao and WHO are 80 µg/m 3 and between 40 -60 µg/m 3 , respectively.The SO 2 annual mean concentrations in 2010, at both the Beth Haim (38.9 µg/m 3 ) and Kas Chikitu (35.6 µg/m 3 ) air stations, was the only year concentrations were less than the recommended guidelines, however, it is important to note the measurements were only for a 7 month period (July-December).The

Potential Risks of SO2 Inhalation
Many epidemiological studies have reported relative risk (RR) estimates, hazard ratios (HR) and odds ratios (OR) for associations between mortality and morbidity and SO 2 , yet results are somewhat inconclusive (Figures 5-7) [27]- [41].
This study primarily focused on literature published within the last five years since there are several meta-analyses and reviews covering literature published prior to 2010.
Many of the studies reported relative risk estimates that were either close to or included unity or had large confidence intervals (Figure 5).A study evaluating air pollution effects on residents of central Taiwan, reported positive relative risk estimates (RR: 1.043, 95% CI: 1.018 -1.098) for associations between SO 2 (12.6 µg/m 3 winter mean concentration) and all-cause mortality during the winter months [27].Similarly, the health impacts were assessed in the megacity of Iran and also reported positive associations (RR: 1.004, 95% CI: 1.003 -1.0048) between all-cause mortality and SO 2 concentrations (89.2 µg/m 3 annual mean) [33].Lai et al. [28] conducted a meta-analysis of research between 1989 and 2010 reporting health effects on Chinese populations in China, Taiwan and Hong Kong, which also resulted in positive associations (RR: 1.007, 95% CI: 1.0045 -1.0097) between SO 2 (14 -213 µg/m 3 annual mean) and all-cause mortality.(15.72 µg/m 3 spring mean) and cardiovascular mortality during the spring when the weather was described as dry and tropical [30].The meta-analysis evaluating air pollution effects on Chinese populations also reported positive associations between short-term SO 2 exposure (14 -213 µg/m 3 annual mean) and cardiopulmonary (RR: 1.01, 95% CI: 1.009 -1.015) and cardiovascular mortality (RR: 1.007, 95% CI: 1.004 -1.01) [28].A meta-analysis of 34 studies concluded statistically significant positive associations (RR: 1.01, 95% CI: 1.003 -1.017) with SO 2 and increased risks of myocardial infarction [31].
The hazard ratios reported for mortality and various respiratory morbidities also illustrated weak and inconsistent associations with SO 2 and various endpoints (Figure 7).The National English Cohort study, consisting of over 800,000 participants, reported statistically significant positive associations (HR: 1.05, 95% CI: 1.01 -1.08) between lung cancer mortality and SO 2 [42].Similarly, a Japanese cohort study, consisting of over 63,000 study participants, also reported significant positive associations (HR: 1.26, 95% CI: were reported in the Japanese cohort study [40] [41]. In summary, the epidemiological studies presented a mix of weak positive associations (i.e., RR, HR, OR < 3.0) between health effects and SO 2 and nonsignificant findings.Potential inconsistencies between studies and results could be due to confounding factors with copollutants and various study designs and methodology, making individual findings difficult to interpret and inconclusive as to the effect of observed levels of SO 2 on health.In general, cardiovascular and respiratory effects and mortality were evaluated in locations with a wide range in annual mean concentrations ranging from 4 to 675 µg/m 3 .
Evidence regarding short-term exposures to SO 2 were inadequate to infer an association with cardiovascular health [43].As well, the available evidence was inadequate to infer an association between long-term SO 2 exposures and respiratory effects (including asthma), cardiovascular effects and mortality.In the selected epidemiological studies evaluated in a USEPA review, associations between short-term SO 2 exposures and some respiratory effects were observed in locations with mean 24-hour average SO 2 concentrations ranging from 2.62 to 78.6 µg/m 3 , with maximum values ranging from 31.4 to 196.5 µg/m 3 .In addition, some associations with mortality were observed with mean 24-hour average SO 2 concentrations less than 26.2 µg/m 3 .that illustrated increasing wind speeds decreased aerosol concentrations which resulted in decreased pollen related hospital admissions [47].However, other studies have demonstrated statistically significant impacts from SO 2 and other emissions downwind of oil and gas exploration and production activities [46].

Meteorological Parameters and SO2 Concentrations
Likewise, this study illustrates that increasing wind speeds were correlated with increasing SO 2 concentrations downwind of the Isla Refineriá's plume trajectory in Curaçao.Furthermore, the previously reported increasing inhalable particulate matter in this region mimics that of the SO 2 suggesting a single source (Figure 9; Pulster, unpublished data).This suggests with increasing wind speeds, pollution plumes and fluxes are also expected to increase, ultimately impacting local populations residing within the refinery's plume trajectory.

Conclusions
The objective of this investigation was to conduct an exposure assessment by [13] [14] [15][16].The controlled experiments in the 1950s examined the effects of SO 2 inhalation using controlled exposure experiments on healthy individuals.These studies revealed considerable inter-individual variability among healthy individuals however bronchoconstriction responses in most were induced at levels approaching 5 ppm (10,480 -13,100 µg/m3

Daily SO 2
mean concentrations were downloaded from two local air stations in Curaçao, Beth Haim (n = 1605) and Kas Chikitu (n = 1622).The 24-hour daily mean concentrations ranged from 0.2 to 449 µg/m 3 and 6.8 to 139 µg/m 3 at the Beth Haim and Kas Chikitu stations, respectively (Figure1).Mean annual concentrations at the Beth Haim station ranged from 38.9 µg/m 3 in 2010 to 170.4 µg/m 3 in 2014 (Figure2).Similar, statistically significant temporal trends were observed at the Beth Haim station with a moderately strong increasing trend (R 2 = 0.86) over time.Mean annual SO2 concentrations at the Kas Chikitu station ranged from 35.6 µg/m 3 in 2010 to 55.5 µg/m 3 in 2014, also illustrating a moderately strong increasing trend (R 2 = 0.86) over time (Figure2).Increasing trends in PM 10 concentrations downwind of the refinery were also reported (Pulster, unpublished data).

Figure 3 .
Figure 3. Global comparison of mean annual SO 2 concentrations (µg/m 3 ).Sources for the concentrations from other countries are as follows †European Environment Agency, ‡USEPA and *Clean Air Asia Citites ACT.Red bars indicate data relevant to current study.

24 -
hour guidelines issued by the European Commission and Curaçao both recommend 125 µg/m 3 with three permissible excursions.Curaçao recommends the mean 24-hour maximum concentrations do not exceed 365 µg/m 3 more than once per year.In addition, WHO recommends 24-hour maximum concentrations of 100 -150 µg/m 3 .The number of days that exceed the 24-hour guidelines at both stations are also increasing over time.The 24-hour recommended SO2 guidelines were within compliance for the majority of 2010 (95%), 2011 (78%) and 2012 (84%) at the Beth Haim Station (Figure 4).However, 2013 and 2014 exceeded the recommended SO 2 guidelines 74% and 81% of the year, respectively.Curaçao allows three excursions per year above the 24-hour maximum recommendation (125 µg/m 3 ).In 2011, 2012, 2013, and 2014 this value was exceeded 30, 15, 40 and 54 times, respectively.In contrast, the daily SO 2 concentrations measured at the Kas Chikitu station were compliant for the majority of each of the years measured in this study [2010 (99%).2011 (98%), 2012 (96%), 2013 (98%) and 2014 (95%)].The only year that exceeded the number of permissible excursions was 2012, with a total of 4 days exceeding 125 µg/m 3 .

Figure 4 .
Figure 4. Percentages of each year that were either in compliance or exceeded current maximum 24-hour guidelines for SO 2 concentrations at the Beth Haim air monitoring station.

Figure 5 .
Figure 5. Relative risk estimates (95% CI) for SO 2 associated mortality and morbidity from published literature.

Figure 8 .
Figure 8. Regression analysis between wind speeds (km/h) and SO 2 concentrations (µg/m 3 ) at the Beth Haim (a) and the Kas Chikitu (b) air monitoring station measurements from 2010 through 2014.
analyzing levels of SO 2 in ambient air surrounding Willemstad, Curaçao in order to determine if any temporal trends exist in the measured concentrations, to verify if measured levels exceed current public health guidelines and to identify potential health risks.In conclusion, concentrations of SO 2 in Curaçao are among the highest reported globally and demonstrate an increasing trend over time.Levels of SO 2 exceeded the annual and 24-hour guidelines recommended by Curaçao, the European Commission, and the World Health Organization.

Figure 9 .
Figure 9. Increasing trends of monthly measurements of SO 2 and PM 10 at the Beth Haim and Kas Chikitu air stations in Curaçao from 2010 through 2014, suggesting one source.
primarily used to monitor the residential load of SO 2 , hydrogen sulfide (H 2 S) and PM 10 .Available hourly and daily measurements of SO 2 were downloaded from June 1, 2010 through December 31, 2014 for analysis from the Kas Chikitu station.Twenty-four hour SO 2 daily means were downloaded from the Beth Haim station (n = 1605) and the Kas Chikitu station (n = 1622).Monitoring stations operate in accordance with the ISO/IEC 17025 accreditation (certificate number L 426) of GGD Amsterdam using ultraviolet fluorescence (Thermo 43i-TLE and Thermo 450i Gas Analyzer) methodology to measure SO 2 .
Two air monitoring stations, Beth Chaim and Kas Chikitu, commissioned by the government of Curaçao have been collecting validated and continuous measurements of air quality parameters (SO 2 , PM 10 , TSP, H 2 S) since mid-2010.The Beth Chaim station is located at the western edge, downwind of the Schottegat industrial area of the refinery and only measures SO 2 and TSP.Kas Chikitu is located approximately 2 -3 km downwind in the Marchena/Wishi residential E. L. Pulster et al.DOI: 10.4236/jep.2018.93014197 Journal of Environmental Protection area and is E. L. Pulster et al.