Refinery Capacity and Carcinogen Emissions: An analysis of pollution and sustainability among US oil refineries

ABSTRACT

Crude oil is a cornerstone of the American energy sector, particularly vital for transportation and plastic manufacturing industries. This study investigates the environmental impacts of the oil industry, focusing on carcinogenic emissions from oil refineries. Using data from the Environmental Protection Agency’s Toxic Release Inventory and the Energy Information Administration’s refinery capacity reports, Kendall rank correlation and regression analyses examine the relationship between refinery capacity and emissions of benzene, ethylbenzene, naphthalene and polycyclic aromatic compounds from 2010 to 2022. The analysis revealed significant findings: benzene emissions in Wyoming showed a statistically significant negative correlation with refinery capacity (τ=−0.49, p<0.05), suggesting that increased capacity correlates with lower benzene emissions. Ethylbenzene emissions in Alaska (τ=0.61, p<0.05) and Texas (τ=0.46, p<0.05) were positively correlated with capacity, indicating that higher production is associated with increased emissions. Naphthalene emissions display mixed results, with a significant positive correlation in Alaska (τ=0.63, p<0.05) and a significant negative correlation in California (τ=−0.50, p<0.05). R-squared values for these relationships suggest moderate explanatory power, such as 0.25 for benzene in Wyoming and 0.19 for ethylbenzene in Alaska. These findings indicate that although regulatory measures and technological advancements have reduced emissions over time, certain carcinogens still pose challenges as production capacity increases. The study underscores the need for stronger regulations and sustainable practices in oil refining to address the ongoing risks to public health and environmental sustainability.

INTRODUCTION

The United States is a major producer of crude oil and plays a vital role in the global energy market. However, its extensive use significantly contributes to carbon pollution and climate change. In addition to carbon emissions, refining crude oil releases carcinogenic compounds into the air, water and land, posing severe health risks to humans and serious environmental challenges. Addressing these emissions is crucial for protecting public health and ensuring environmental sustainability. Advancements in emission control technologies and sustainable practices in oil refining are essential to mitigate carcinogenic impacts.

LITERATURE REVIEW

The modern oil industry in the United States began in Pennsylvania in 1859, when Edwin Drake successfully drilled the first commercial oil well near Titusville (Black, 2020; Vassiliou and Mir-Babayev, 2022; Webb and Fee, 2024). Drake’s pioneering method of using a steam engine to drill for oil spurred rapid growth in the industry during the late 1800s (Craig, 2020). Major discoveries of oil were later made in California and Texas, which boosted production and triggered the oil age (McNally, 2017; Hall and Klitgaard, 2018; Meierding, 2020). World wars have increased global dependency on oil, leading to advances in drilling, refining and distribution (Douet, 2020; Johnstone and McLeish, 2020; Van de Graaf and Sovacool, 2020). Beyond energy, oil revolutionised agriculture through the creation of fertilisers and pesticides and remains critical for transportation (Rahman et al., 2015; Jungers et al., 2022).

The formation of the Organization of the Petroleum Exporting Countries (OPEC) in 1960 significantly influenced global oil markets, leading to an oil embargo in the 1970s that caused oil prices to skyrocket (Brew, 2019; Pirani, 2022). This crisis had a major impact on the US automobile industry, exposing the nation’s dependence on foreign oil and prompting efforts to boost domestic oil production (Venn, 2016; Ediger and Berk, 2018; Zulkifli and Haqeem, 2022). In the mid-2010s, techniques such as hydraulic fracking and horizontal drilling revolutionised the US oil industry, reducing market risks and increasing returns, leading to a modern transition from being a significant oil importer to the world’s leading crude oil producer by 2018 (Bourghelle et al., 2021; Carson, 2022; Çakmak and Acar, 2022). As of 2021, the domestic oil and natural gas industry accounted for about 7.6% of the national GDP (PwC, 2023). Approximately 65.59% of domestic oil is used for household applications and is an important component of plastic and petroleum products like gasoline and diesel, which fuel the transportation sector (Wu and Chen 2019; Conway and Robertson, 2021).

Despite advances in EV technology, global EV sales slowed in 2023, growing only 29.8% compared to 54.2% in 2022 due to factors like high costs and limited charging infrastructure (Alanazi, 2023; Chidambaram et al., 2023; Iliff, 2024; Lu, 2024). The economic advantages of crude oil, such as its low costs, have hindered the adoption of biodegradable plastics, emphasising the continued relevance of crude oil (Guan et al., 2021; Moshood et al., 2021; Singh et al., 2022; Shaikh et al., 2024).

Oil refineries play a crucial role in the oil industry, transforming crude oil into valuable products such as gasoline, diesel and jet fuel through a series of physical and chemical processes (Demirbas and Bamufleh, 2017; Gudde, 2018; Adebiyi, 2022). The capacity of oil refineries is measured in terms of the number of barrels a refinery can produce per day (Morrow III et al., 2015). Large refineries especially benefit from economies of scale, producing higher quantities and minimising operational costs (Bruins and Sanders, 2012; Khor and Varvarezos, 2017; Álvarez et al., 2018). By optimising their operations, refineries can sustainably meet the demand for oil products (Yatimi et al., 2024). Despite a decrease in the number of refineries since 1985, the US refining industry, the largest and most advanced in the world, has increased its capacity through consolidations and investments, growing from 14,000 barrels per day in 1985 to 20,000 barrels per day in 2019 (Melek and Ojeda, 2017; Ruble, 2019).

The Energy Information Administration (EIA) collects annual capacity data for refineries in the US and plays a key role in gathering and distributing energy information for policymaking (Forman et al., 2014; Nost et al., 2021; Nalley and LaRose, 2022). It also raises public awareness of the relationship between energy, the economy and the environment (Enríquez-de-Salamanca, 2021). Additionally, the EIA’s aggregate capacity data are utilised to assess Criteria Air Pollutants and Greenhouse Gas Emissions (GHG) (Sun et al., 2019; Madugula et al., 2021; Rahi et al., 2021).

The Environmental Protection Agency (EPA) sets air quality standards and regulates hazardous air pollutants impacting respiratory health (Brumberg and Karr, 2021). The Clean Air Act and Clean Water Act (CWA) regulate industrial air and water pollution, including from oil refineries (Clayton, 2015; Samanta and Mitra, 2021; Shafiq et al., 2022; Kim et al., 2023; Ozymy et al., 2023; Babich, 2024). Key CWA components include total maximum daily loads (TMDL), the National Pollutant Discharge Elimination System (NPDES) and priority pollutants, all overseen by the EPA (Pradhan et al., 2014; Kumar et al., 2020; Anica and Elbakidze, 2023). TMDL limits pollutants in water, while NPDES requires facilities to report water pollutants (Dulle, 2023; Rai et al., 2024). The NPDES regulates priority pollutants such as metals, polycyclic aromatic hydrocarbons (PAHs) and volatile organic compounds (VOCs) to protect water quality and health, although it faces challenges in addressing newer contaminants like per- and polyfluoroalkyl substances due to data limitations (Borgens, 2019; Gatz, 2019)

The release of toxic chemicals from oil refinery operations pollutes water, land and air, posing risks to human and environmental health (Mohebian et al., 2021; Radelyuk et al., 2021; Wu et al., 2022). Proximity to refineries significantly increases the risk of various cancers and advanced metastatic diseases (Domingo et al., 2020; Williams et al., 2020; Onyije et al., 2021). According to the government of California, living near refineries exposes residents to increased risks of asthma, cancers, birth defects and other chronic health issues (Orozco, 2021). Andersson et al. (2024) identified an increased risk of leukaemia among refinery workers exposed to benzene, a known carcinogen. Other harmful substances, including ethylbenzene, naphthalene and polycyclic aromatic compounds, also pose significant health risks, alongside wastewater from refineries, which has been shown to harm ecosystems and elevate cancer risks for nearby communities (Ruckart et al., 2015; Varjani et al., 2017; Singh and Shikha, 2019; Molaei et al., 2020; Khoshakhlagh et al., 2023; Barbeş et al., 2024; Wang et al., 2025). Soil is negatively affected, with pollutants altering its properties, reducing water permeability and impacting plant development (Kuzhaeva and Berlinskii, 2018). Air pollution is the most significant issue, as boilers and combustion release carbon monoxide (CO) and sulphur oxides (SOx), causing respiratory irritation (Islam and Mostafa, 2021). The reduced industrial activities during the COVID-19 lockdowns, which led to improved air quality, highlight the impact of industrial emissions on pollution levels (Damiani et al., 2024; Singh et al., 2024a).

The carcinogenicity of these compounds is critical in industrial settings. PAHs, such as benzo[a]pyrene (BaP), play a significant role in increasing cancer risk (Aquilina and Harrison, 2023; Ravanbakhsh et al., 2023). Pozo et al. (2023) observed that BaP-equivalent concentrations ranged from 2 to 108 ng/m³ in central Chile, with higher levels during colder months due to increased combustion sources, highlighting the impact of seasonal factors on exposure. Naphthalene shows higher acute toxicity, with an LC50 of 63.6 µL/L air compared to benzene’s 115.9 µL/L air (Pajaro-Castro et al., 2017). However, benzene presents the highest long-term cancer risk, with 264.1 cases per million compared to ethylbenzene’s 217.9 cases per million (Saeedi et al., 2024). Its cancer unit risk is approximately 10 times higher than that of ethylbenzene, with a US EPA/IARC classification of A versus ethylbenzene’s 2B (Dehghani et al., 2020). Long-term benzene exposure is particularly concerning due to its association with increased risks of leukaemia and other severe chronic health effects.

Research findings highlight the lack of regulation of discharges from oil refineries. In 2020, ‘thirteen refineries exceeded EPA’s “action level” … reporting annual benzene concentrations that range from 9.36 micrograms to more than 31 micrograms for the year’ into the air (Kunstman et al., 2021, p. 3). Markow et al. (2023) reported, ‘Almost 83 percent of refineries exceeded their permitted limits on water pollutants at least once between 2019 to 2021. But only about a quarter of the refineries with violations were penalised during this period’ (p. 8). Stricter enforcement and public oversight are essential to mitigate the environmental and societal harm caused by the ecological footprint and emissions from oil refineries (Khan et al., 2021; Griffiths et al., 2022; Parashar and Thakur, 2024).

MATERIAL AND METHODS

The Toxic Release Inventory (TRI) is a database compiled by the EPA that tracks the management and release of chemicals known to be hazardous to human health (Pavan et al., 2023; Swenson, 2024). Facilities are obligated to report to the EPA for inclusion in the TRI if they ‘produce or process more than 25,000 pounds or otherwise use more than 10,000 pounds of toxic chemicals’ (Pham and Roach, 2024, p. 454).

This study analysed EIA data from one refinery in each of the top oil-producing states: Alaska, California, New Mexico, North Dakota, Texas and Wyoming (EIA, 2023). These refineries report their TRI emissions to the EPA (EIA, 2024; see Table 1).

Table 1 Refineries considered

Alaska	Tesoro Alaska- Kenai Refinery
California	Chevron Products CO. DIV of Chevron USA INC.
New Mexico	HollyFrontier Navajo Refining LLC- Artesia Refinery
North Dakota	Marathon Mandan Refinery
Texas	Deer Park Refining LLC
Wyoming	HF Sinclair Parco Refining CO LLC

For simplicity, the references to refineries are the state names and specific refinery names. For example, Alaska-Tesoro Alaska-Kenai Refinery, California-Chevron Products CO. DIV of Chevron USA INC., New Mexico-HollyFrontier Navajo Refining LLC-Artesia Refinery, North Dakota-Marathon Mandan Refinery, Texas-Deer Park Refining LLC and Wyoming-HF Sinclair Parco Refining CO LLC. This approach aims to provide geographic context and specific facility identification throughout the analysis.

The corresponding NAICS codes for these petroleum refineries did not contain secondary NAICS codes in all years, unlike other similar facilities that often included secondary NAICS codes, such as natural gas distribution and petrochemical manufacturing. As such, this study accurately measured the oil refining process of major facilities.

The EPA (2024a) defines total on-site releases as the sum of air, land and water discharges occurring at the facility. This study uses total on-site values from the TRI database for 2010–2022 (see Table 2 for Alaska and Table A1 for all facilities). Dioxin, reported in grams (g), was converted to pounds (lb.) before summing the total on-site releases.

Table 2 2010 Alaska Kenai Refinery total on-site releases

10. FACILITY NAME	11. FACILITY STREET	12. FACILITY CITY	15. FACILITY ZIP CODE	41. PRIMARY NAICS CODE	71. PARENT COMPANY NAME	78. CHEMICAL NAME	83. CLEAN AIR ACT IND	84. CARCINOGEN IND	218. TOTAL ON-SITE RELEASES
TESORO ALASKA-KENAI REFINERY	3600 KENAI SPUR HWY	KENAI	99611	324110	MARATHON PETROLEUM CORP	Benzene	YES	YES	5932
TESORO ALASKA-KENAI REFINERY	3600 KENAI SPUR HWY	KENAI	99611	324110	MARATHON PETROLEUM CORP	Carbon disulfide	YES	NO	840
TESORO ALASKA-KENAI REFINERY	3600 KENAI SPUR HWY	KENAI	99611	324110	MARATHON PETROLEUM CORP	Cyclohexane	NO	NO	4294
TESORO ALASKA-KENAI REFINERY	3600 KENAI SPUR HWY	KENAI	99611	324110	MARATHON PETROLEUM CORP	Mercury compounds	YES	NO	4
TESORO ALASKA-KENAI REFINERY	3600 KENAI SPUR HWY	KENAI	99611	324110	MARATHON PETROLEUM CORP	Polycyclic aromatic compounds	YES	YES	27
TESORO ALASKA-KENAI REFINERY	3600 KENAI SPUR HWY	KENAI	99611	324110	MARATHON PETROLEUM CORP	Toluene	YES	NO	8426
TESORO ALASKA-KENAI REFINERY	3600 KENAI SPUR HWY	KENAI	99611	324110	MARATHON PETROLEUM CORP	Ethylbenzene	YES	YES	1433
TESORO ALASKA-KENAI REFINERY	3600 KENAI SPUR HWY	KENAI	99611	324110	MARATHON PETROLEUM CORP	Lead compounds	YES	NO	21
TESORO ALASKA-KENAI REFINERY	3600 KENAI SPUR HWY	KENAI	99611	324110	MARATHON PETROLEUM CORP	Methanol	YES	NO	150
TESORO ALASKA-KENAI REFINERY	3600 KENAI SPUR HWY	KENAI	99611	324110	MARATHON PETROLEUM CORP	Naphthalene	YES	YES	356
TESORO ALASKA-KENAI REFINERY	3600 KENAI SPUR HWY	KENAI	99611	324110	MARATHON PETROLEUM CORP	Xylene (mixed isomers)	YES	NO	7878
TESORO ALASKA-KENAI REFINERY	3600 KENAI SPUR HWY	KENAI	99611	324110	MARATHON PETROLEUM CORP	1,2,4-Trimethylbenzene	NO	NO	1680
TESORO ALASKA-KENAI REFINERY	3600 KENAI SPUR HWY	KENAI	99611	324110	MARATHON PETROLEUM CORP	Ammonia	NO	NO	13256
TESORO ALASKA-KENAI REFINERY	3600 KENAI SPUR HWY	KENAI	99611	324110	MARATHON PETROLEUM CORP	Benzo[g,h,i]perylene	YES	NO	6
TESORO ALASKA-KENAI REFINERY	3600 KENAI SPUR HWY	KENAI	99611	324110	MARATHON PETROLEUM CORP	Carbonyl sulfide	YES	NO	460
TESORO ALASKA-KENAI REFINERY	3600 KENAI SPUR HWY	KENAI	99611	324110	MARATHON PETROLEUM CORP	Cumene	YES	YES	24
TESORO ALASKA-KENAI REFINERY	3600 KENAI SPUR HWY	KENAI	99611	324110	MARATHON PETROLEUM CORP	n-Hexane	YES	NO	17604

Note: Source: 2010 TRI Database

For this study, only benzene, ethylbenzene, naphthalene and polycyclic aromatic compounds were considered as the sample set of harmful carcinogens for analysis. The amounts were reported in annual lb. for all facilities selected (see Table 3 for Alaska; see Appendix A2, A3, A4, A5, A6 for the other facilities in this study).

Table 3 Alaska selected carcinogens total on-site releases (lb.)

Carcinogens	2010	2011	2012	2013	2014	2015	2016	2017	2018	2019	2020	2021	2022
Benzene	5,932	5,013	3,716	4,292	1,621	3,764	3,031	2,254	2,569	2,925	3,107	2,904	3,404
Ethylbenzene	1,433	1,172	824	830	537	482	530	545	569	698	846	782	942
Naphthalene	356	275	95	221	144	144	175	224	252	256	244	234	230
Polycyclic aromatic compounds	27	24	20	23	23	25	27	29	28	29	263	28	28

To assess carcinogen release in the refining process, this study used the annual capacity of refineries, provided in units of barrels per calendar day (b/cd), to determine production sustainability (see Table 4).

Table 4 Refinery daily capacity (b/cd)

Years	Alaska	California	Wyoming	New Mexico	North Dakota	Texas
2010	68,000	273,000	74,000	105,000	58,000	327,000
2011	68,000	276,000	74,000	105,000	60,000	327,000
2012	65,000	269,000	74,000	105,000	68,000	327,000
2013	65,000	269,000	74,000	105,000	70,000	327,000
2014	65,000	269,000	85,000	102,000	73,860	316,600
2015	58,500	269,000	85,000	102,000	73,860	285,500
2016	62,700	269,000	85,000	98,000	73,800	325,700
2017	62,700	269,000	75,000	110,000	71,000	275,000
2018	68,000	269,000	75,000	110,000	71,000	275,000
2019	68,000	269,000	75,000	110,000	71,000	318,000
2020	68,000	269,000	75,000	110,000	71,000	302,800
2021	68,000	269,000	75,000	110,000	71,000	312,500
2022	68,000	269,000	75,000	110,000	71,000	312,500

Source: EIA Refinery Capacity Report

The annual capacities for each of the refineries were calculated as shown in Formula 1. The results are presented in Table 5 (see Table A7 for raw annual capacity). Based on previous studies that used the weight of barrels in lb., a single barrel is considered to weigh approximately 302.82 lb. This value was multiplied by 365 to convert daily capacity into annual capacity in pounds, reflecting the number of days per year (Zabawski, 2015; Elsharafi et al., 2017).

Table 5 Annual capacity in billions of lb. (Rounded to one decimal place)

States	2010	2011	2012	2013	2014	2015	2016	2017	2018	2019	2020	2021	2022
Alaska	7.5	7.5	7.2	7.2	7.2	6.5	6.9	6.9	7.5	7.5	7.5	7.5	7.5
California	30.2	30.5	29.7	29.7	29.7	29.7	29.7	29.7	29.7	30	30	30	30
New Mexico	11.6	11.6	11.6	11.6	11.3	11.3	10.8	12.2	12.2	12	12	12	12
North Dakota	6.4	6.6	7.5	7.7	8.2	8.2	8.2	7.8	7.8	7.8	7.8	7.8	7.8
Texas	36.1	36.1	36.1	36.1	35	31.6	36	30.4	30.4	35	34	35	35
Wyoming	8.2	8.2	8.2	8.2	9.4	9.4	9.4	8.3	8.3	8.3	8.3	8.3	8.3

Formula 1:

  

$$\begin{array}{l}Annual capacity {\left(lb.\right)}_{refinery, year}=\\ Barrel per calendar day {\left(\frac{b}{cd}\right)}_{refinery, year}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}*365*302.82\end{array}$$

To understand the environmental impact of carcinogenic emissions from refineries, the annual emissions of these carcinogens per pound of capacity were determined by dividing the total on-site emissions for each carcinogen by the annual capacity of the refinery (see Formula 2). The normalised (weighted emissions) values provide insight into these two variables in terms of the environmental impacts of carcinogens.

Formula 2:

  

$$\begin{array}{l}Normalized carcinigen {\left(lb.\right)}_{refinery, year}=\\ \frac{Carcinogen released {\left(lb.\right)}_{refinyery, year}}{Annual capacit{y}_{refinery, year}}\end{array}$$

To analyse the relationship between annual refinery capacity and carcinogenic emissions, statistical tests were applied to more accurately study the relationship. The Shapiro–Wilk test was used to test the normality of the data for total annual carcinogen emissions (by chemical) in terms of total annual capacity, which steered the selection of the appropriate correlation test (González-Estrada et al., 2022). The capacity data for all refineries revealed a p-value<0.05, suggesting that the data were not normally distributed. Given the non-normal distribution of one variable and the small sample size, the Kendall rank correlation coefficient test was deemed ideal for analysing the relationship between annual capacity and annual carcinogen emissions (Shiekh and El-Hashash, 2022).

The R-squared and slope values for normalised emissions were calculated as an additional experimental design to provide clarity into how well the model explains the direction, efficiencies, key trends and variability in emissions over time (Ozili, 2023). All quantitative analyses were performed using the pandas, scipy.stats and statsmodels.api packages in Python (version 3.12.0); these are commonly used for data manipulation, statistical analysis and statistical modelling (Haslwanter, 2016; Rahaman et al., 2022).

RESULTS AND DISCUSSION

Fig. 1 depicts the summed normalised annual emissions of carcinogenic compounds across all states over the period from 2010 to 2022.

Fig. 1 Summed normalized annual emission of carcinogens

Benzene emissions show a significant decline from 2010 to 2012, followed by a brief peak in 2013. After aggregately dropping in 2014, emission levels stabilised with slight fluctuations, including a slight increase in 2022. Ethylbenzene emissions declined sharply from 2010 to 2012, increased in 2013, declined in 2014 and stabilised until 2018. In 2019, ethylbenzene emission levels exhibited a slight upwards trend, with a noticeable rise in 2022. In contrast, naphthalene emissions remain relatively low and stable throughout the entire period, with only minor fluctuations. Polycyclic aromatic compounds also maintain low emission levels, displaying almost constant levels with very slight fluctuations from 2010 to 2022.

The reduction of benzene, ethylbenzene, naphthalene and polycyclic aromatic compounds (PACs) resulted from the implementation of the MSAT regulations (EPA, 2007). To comply, refineries made process adjustments such as routing hydrocarbon-rich streams, including those containing ethylbenzene and naphthalene, to reformers and utilising benzene saturation units. These adjustments not only targeted benzene but also contributed to the overall reduction of these hazardous compounds in refinery emissions. Conversely, the increasing trend of carcinogens from 2021 to 2022 may be attributed to post pandemic operations, as restrictions were lifted, thus resuming daily production (Sanda et al., 2023; Singh et al., 2024b).

Table 6 shows the statistical outputs for each facility and the selected chemicals reported from these facilities. The Kendall tau correlation and regression analysis revealed few significant values for specific chemicals and facilities. These results highlight the relationship between emissions, annual capacity and time, providing insights into the effectiveness of controlling harmful carcinogen emissions. Significant findings with a p-value<0.05 are highlighted in red.

Table 6 Python statistical outputs

State	Chemical	SW p-value (Carcinogen)	SW p-value (Capacity)	Kendall (τ)	τ p-value	Normalized slope	R-squared
Alaska	Benzene	>0.05	<0.05	0.08	>0.05	−2.46E-08	0.39
California	Benzene	<0.05	<0.05	0.12	>0.05	−1.86E-10	0.01
New Mexico	Benzene	<0.05	<0.05	−0.03	>0.05	−1.43E-07	0.44
North Dakota	Benzene	>0.05	<0.05	0.06	>0.05	5.26E-09	0.11
Texas	Benzene	>0.05	<0.05	0.03	>0.05	−2.58E-09	0.01
Wyoming	Benzene	<0.05	<0.05	−0.49	<0.05	−1.02E-07	0.25
Alaska	Ethylbenzene	>0.05	<0.05	0.61	<0.05	−3.97E-09	0.19
California	Ethylbenzene	<0.05	<0.05	−0.21	>0.05	1.47E-08	0.2
New Mexico	Ethylbenzene	<0.05	<0.05	0.06	>0.05	−1.19E-07	0.43
North Dakota	Ethylbenzene	>0.05	<0.05	0.23	>0.05	3.69E-09	0.1
Texas	Ethylbenzene	<0.05	<0.05	0.46	<0.05	−1.12E-08	0.22
Wyoming	Ethylbenzene	<0.05	<0.05	−0.31	>0.05	1.74E-08	0.11
Alaska	Naphthalene	>0.05	<0.05	0.63	<0.05	−2.25E-11	0.0001
California	Naphthalene	>0.05	<0.05	−0.5	<0.05	2.76E-10	0.24
New Mexico	Naphthalene	<0.05	<0.05	0.09	>0.05	−6.48E-09	0.38
North Dakota	Naphthalene	>0.05	<0.05	−0.32	>0.05	−1.54E-08	0.76
Texas	Naphthalene	<0.05	<0.05	−0.24	>0.05	3.31E-10	0.001
Wyoming	Naphthalene	>0.05	<0.05	−0.1	>0.05	−3.97E-10	0.07
Alaska	Polycyclic aromatic compounds	<0.05	<0.05	0.32	>0.05	7.43E-10	0.11
California	Polycyclic aromatic compounds	>0.05	<0.05	−0.12	>0.05	−2.54E-11	0.05
New Mexico	Polycyclic aromatic compounds	<0.05	<0.05	0.06	>0.05	−1.88E-10	0.22
North Dakota	Polycyclic aromatic compounds	>0.05	<0.05	−0.38	>0.05	−1.01E-09	0.38
Texas	Polycyclic aromatic compounds	<0.05	<0.05	−0.35	>0.05	−1.77E-11	0.002
Wyoming	Polycyclic aromatic compounds	<0.05	<0.05	−0.12	>0.05	8.44E-10	0.09

As seen in Table 6, benzene emissions in Wyoming exhibited a statistically significant negative correlation (τ=−0.49, p<0.05), suggesting higher refinery capacities may be associated with reduced benzene emissions. The regression analysis further supports this finding, as the normalised slope of −1.02E-07 and R-squared value of 0.25 indicate a moderate fit and decreasing trend in benzene emissions over time.

For Alaska, ethylbenzene emissions showed a statistically significant positive correlation with annual capacity (τ=0.61, p<0.05), indicating that higher capacities are linked with increased ethylbenzene emissions. The regression analysis results for the normalised slope of −3.97E-09 and R-squared value of 0.19 suggest a reduction in emissions over time. Naphthalene emissions in Alaska also exhibited a statistically significant positive correlation with capacity (τ=0.63, p<0.05), with a normalised slope of −2.25E-11 but a low R-squared value of 0.0001.

California’s naphthalene emissions display a statistically significant negative correlation (τ=−0.50, p<0.05), further implying that higher capacities may be linked with reduced emissions. The regression analysis showed a normalised slope of 2.76E-10 and an R-squared value of 0.24, indicating an increasing trend in emissions over time.

Other significant findings include a statistically significant positive correlation of ethylbenzene emissions with capacity in Texas (τ=0.46, p<0.05), with a slope of −1.12E-08 and an R-squared value of 0.22. This indicates a complex relationship, as emissions have been decreasing over time even though increased capacity typically leads to higher emissions of ethylbenzene.

The analysis revealed a significant positive correlation between annual carcinogen emissions and the increased capacity of oil refineries. In Alaska, there is a significant positive correlation between ethylbenzene emissions and refinery capacity, indicating that despite increased production capacity, systematic emissions controls or other factors have effectively reduced ethylbenzene emissions. Similarly, naphthalene emissions in Alaska are positively correlated with capacity, but the low R-squared value suggests that other factors besides production capacity have influenced the reduction in emissions over time. In 2016, a significant EPA and Department of Justice settlement with Tesoro required the Kenai refinery to adopt flare gas recovery technology and leak detection systems to monitor and suppress VOC emissions (Parker, 2016). This event highlights how regulatory enforcement and technological upgrades played a crucial role in reducing harmful emissions from the Kenai refinery, even with increased production capacity.

In Texas, a significant positive correlation exists between ethylbenzene emissions and refinery capacity, reflecting ongoing efforts to improve emissions controls and reduce environmental impacts as refinery capacity increases. Overall, the positive correlations indicate a relationship between increased capacity and emissions, but effective emission control measures and other phenomena significantly mitigate these emissions, demonstrating that the relationship is not simply one of direct proportionality.

However, in California, naphthalene emissions exhibit a significant negative correlation with capacity, suggesting that larger refinery capacities are associated with lower emissions at a given time. Despite this, the overall trend shows a slight increase in emissions over time, as indicated by the positive slope in the regression analysis. This increase could be attributed to the fact that naphthalene, as identified by OEHHA, requires specialised laboratory analysis and air monitoring methods that are not currently available, limiting accurate emission control and tracking (CARB and CAPCOA, 2019).

Similarly, there is a significant negative correlation between benzene emissions and refinery capacity in Wyoming, indicating that as refinery capacity increased, emission controls became more efficient, leading to reduced benzene emissions. Notably, the normalised emission data show a decreasing slope regardless of the direction of the correlation, highlighting that emissions per unit of capacity are generally decreasing. This trend highlights the impact of improved control measures, such as the implementation of stricter Benzene Waste Operations NESHAP regulations following the 2008 Consent Decree, which mandated more effective management of benzene-containing waste at the Sinclair Wyoming Refinery (EPA, 2024b). These regulations require rigorous monitoring and waste-handling processes, which directly contribute to the overall reduction in benzene emissions. Further examination could explore whether this decrease is driven by the implementation of sustainability initiatives, stricter state regulations enforced by the Wyoming Department of Environmental Quality or the EPA’s consent decrees, which compel refineries to adopt advanced emissions reduction technologies and comply with stricter pollution standards.

Overall, these trends and correlations illustrate the relationship between refinery capacity and carcinogen emissions from 2010 to 2022. The decreasing emission patterns related to production for several facilities suggest progress in emission reduction efforts and sustainable production of crude oil products. Of note, the decline in benzene emissions may be explained by effective regulatory measures such as the 2015 EPA rule, which required refineries ‘to monitor benzene levels along the perimeter of their facilities’ (Kunstman et al., 2020, p. 5). However, as mentioned by Kunstman et al. (2021), several facilities have recently exceeded benzene air emission levels, indicating a slight increase in weighted emissions since 2021. This pattern might signify the need for stronger regulations and/or enforcement for carcinogens that have increased since that period. Conversely, the stable levels of weighted emissions of naphthalene and PACs suggest effective current measures. These approaches used in controlling these carcinogens could serve as models for sustainable manufacturing, aiming to stabilise or reduce other carcinogens released from these facilities.

While incorporating net production data would provide a more detailed understanding of the relationship between production and emissions, such data were not available for individual refineries during the period of this study. Therefore, production capacity was used as a proxy to estimate operational levels. Future studies with access to specific production figures may yield more precise insights. Additionally, exploring technological advancements in emission control systems and sustainable production methodologies can offer deeper insights into overall emission reduction practices (Suku et al., 2024). Further inquiry should also explore domestic and international regions and refineries that practice sustainable crude oil refining or require regulatory attention to provide a comparative analysis of carcinogen emissions. While improvements in decreasing carcinogens were evident between 2010–2014, more recent trends highlight the need for a renewed focus on emission control strategies for benzene and ethylbenzene to mitigate their adverse environmental and human impacts.

ACKNOWLEDGMENTS

This work was supported by Purdue University as part of the project under Discovery Park Undergraduate Research Internship (DUIRI).

CONFLICT OF INTEREST

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

SUPPLEMENTARY MATERIAL

Table A1, Summarized total on-site releases; Table A2, California Selected Carcinogens Total On-Site Releases; Table A3, New Mexico Selected Carcinogens Total On-Site Releases; Table A4, North Dakota Selected Carcinogens Total On-Site Releases; Table A5, Texas Selected Carcinogens Total On-Site Releases; Table A6, Wyoming Selected Carcinogens Total On-Site Releases; Table A7, Raw Annual Capacity.

This material is available on the Website at https://doi.org/10.5985/emcr.20240022.

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