Air Pollution Control: Mitigation Hierarchy For Emission Sources

Air pollution is a pressing issue that poses a threat to human health and the environment. To address this, various regulatory frameworks and control measures have been established to mitigate the emission of harmful pollutants into the atmosphere. One such concept is the air pollutant emission control hierarchy, which provides a structured framework for managing and reducing air pollution. This hierarchy categorizes pollutants, sets standards, and outlines strategies for compliance and enforcement, with the ultimate goal of protecting public health and minimizing environmental degradation.

Regulatory standards for multiple pollutants

Regulatory standards for air pollutants are set by the US Environmental Protection Agency (EPA) under the Clean Air Act (CAA). The CAA requires the EPA to regulate emissions of hazardous air pollutants, also known as "air toxics", from categories of industrial facilities in two phases.

In the first "technology-based" phase, the EPA develops standards for controlling the emissions of air toxics from sources in an industry group. These Maximum Achievable Control Technology or MACT standards are based on emissions levels that are already being achieved by the best-controlled and lowest-emitting sources in an industry. MACT standards are intended to achieve emission reductions in a timely manner. New source standards are based on the top-performing similar source, while for existing sources, the EPA must set standards that require at least the level of performance achieved by the average of the top-performing 12% of similar sources.

Within eight years of setting the MACT standards, the CAA directs the EPA to assess the remaining health and environmental risks from each source category in the second regulatory phase, known as the residual risk program. The EPA must determine whether the MACT standards protect public health and the environment adequately and set additional standards if necessary. Every eight years after setting the MACT standards, the CAA also requires the EPA to review and revise the standards to account for improvements in air pollution controls and/or prevention.

The CAA further requires all areas of the country to meet or strive to comply with the National Ambient Air Quality Standards (NAAQS). The EPA must designate areas as meeting (attainment) or not meeting (nonattainment) the standard, and states are required to develop plans to attain and maintain the NAAQS in their areas. The New Source Review (NSR) program is a preconstruction review process for new and modified stationary sources. The CAA also mandates controls on air pollution from mobile sources, including vehicle emission limits for hydrocarbons, carbon monoxide, nitrogen oxides, and particulates.

Monitoring and modelling data

The MPCA employs various methods to gather data and gain insights into the levels and impacts of these pollutants. One of their key approaches is monitoring specific sites within the state. For instance, they have monitoring sites in the Twin Cities and Duluth, where they closely observe air toxics levels. By collecting emissions data from these locations, they can then use modelling techniques to estimate the concentrations of air toxics in other areas across Minnesota. This modelling process is essential for understanding the broader impact of air toxics and identifying areas that may require further investigation or remediation.

The MPCA's monitoring and modelling efforts have identified three critical types of air toxics prevalent in Minnesota. The first of these is Volatile Organic Compounds (VOCs), which are released from a diverse range of sources. These include paint, solvents, adhesives, gasoline, cleaning products, and fuel combustion. Formaldehyde, a specific type of VOC, is frequently found at levels exceeding health benchmarks in urban areas, underscoring the importance of continued monitoring.

The second type of air toxic of concern in Minnesota is metals. These are emitted primarily from industrial processes and the combustion of fossil fuels. The MPCA has found that metals like chromium, arsenic, and nickel sometimes surpass health benchmarks in areas with significant particle pollution. This discovery highlights the need to regulate and reduce metal emissions to protect human health and the environment.

Lastly, Polycyclic Aromatic Hydrocarbons (PAHs) are another class of air toxics monitored by the MPCA. PAHs consist of over 100 chemicals, with sources such as tobacco smoke, wood smoke, vehicle emissions, and asphalt roads. By studying these PAHs and their sources, the MPCA can develop strategies to mitigate their impact and improve air quality for Minnesotans.

In addition to its state-specific initiatives, the MPCA also participates in broader programmes such as the EPA's Urban Air Toxics Monitoring Program. This involvement ensures that the agency remains at the forefront of air quality management and can contribute to national efforts to understand and address air pollution.

Hazardous air pollutants

The EPA's approach to understanding and regulating HAPs involves studying the regulations for industrial sources of air toxics. The EPA also added 1-bromopropane (also known as 1-BP, n-propyl bromide, and nPB) to the list of HAPs. Other additions include individual pollutants classified under the Polycyclic Organic Matter (POM) group, individual isomers of Lindane, and salts and esters of 2,4-D (2,4-Dichlorophenoxy Acetic Acid).

The Minnesota Pollution Control Agency (MPCA) has also added new PFAS pollutants to the list of HAPs. PFAS are per- and polyfluoroalkyl substances, and the MPCA added 428 of these to the list of reportable pollutants due to their potential inhalation risk. The MPCA also collects data on emissions of air toxics from major sources in Minnesota every three years, reporting its findings in the Minnesota Air Toxics Emissions Inventory.

The Mississippi Department of Environmental Quality (MDEQ) implements hazardous air pollutant regulations that include Maximum Achievable Control Technology (MACT) standards and other National Emission Standards for Hazardous Air Pollutants (NESHAP). MDEQ provides technical assistance for the requirements of regulations and has conducted ambient air monitoring for hazardous air pollutants at various locations throughout the state from 2001 to 2013. Specific monitoring efforts have included the EPA's Urban Air Toxics Monitoring Program, an evaluation of ambient air quality impacts following Hurricane Katrina, and participation in the EPA's Air Toxics in Schools Study.

National Emission Standards

The National Emission Standards for Hazardous Air Pollutants (NESHAP) are air pollution standards issued by the United States Environmental Protection Agency (EPA). The standards are authorized by the Clean Air Act and are for pollutants not covered by the National Ambient Air Quality Standards (NAAQS) that may cause an increase in fatalities or serious, irreversible, or incapacitating illness.

NESHAP standards are authorized by Section 112 of the 1970 Clean Air Act, and the regulations are published in the Code of Federal Regulations (CFR). The EPA regulates hazardous air pollutants with Maximum Achievable Control Technology (MACT) standards. These MACT standards require the maximum degree of emission reduction that the EPA determines to be achievable for a particular source category.

The EPA has developed national enforcement initiatives that focus on significant environmental risks and non-compliance patterns. For Fiscal Years 2014 to 2016, the Cutting Hazardous Air Pollutants National Initiatives Strategy focused on categories of sources that emit hazardous air pollutants. The EPA conducts inspections of facilities subject to the regulations to determine compliance. These inspections include checking processes with emission points against design and work practice standards.

Sources subject to NESHAPs are required to perform an initial performance test to demonstrate compliance. To maintain continuous compliance, sources are generally required to monitor control device operating parameters, which are established during the initial performance test. Sources may also need to install and operate continuous emission monitors.

NESHAP rules for source categories are outlined in 40 CFR Part 63, which includes guidance for paint stripping and miscellaneous surface coating operations. The EPA also provides free, non-regulatory, confidential, and on-site technical assistance through the Mississippi Small Business Environmental Assistance Program (SBEAP).

Multi-pollutant approach

Air masses always contain many pollutants in differing amounts, depending on the types of emission sources and atmospheric conditions. Humans are simultaneously exposed to a complex mixture of air pollutants. As such, many organizations have encouraged moving towards a "multi-pollutant approach to air quality".

The multi-pollutant approach is a shift from the historical focus on single-pollutant research. Single-pollutant research is easier to conduct, and the results are more clearly interpretable. Most air quality policies to protect human health are based on single-pollutant strategies. For example, under the Clean Air Act, the U.S. Environmental Protection Agency (EPA) has the responsibility for setting a separate air quality standard for each criteria pollutant.

However, the multi-pollutant approach can save money and time, and achieve significant health, environmental, and economic benefits, while reducing costs and burdens on sources of air pollution. Many pollution sources, such as industrial facilities, emit several different pollutants that directly cause health and environmental impacts or react in the environment to form other harmful pollutants. Some control technologies can reduce emissions of multiple pollutants. It can be more efficient to develop integrated control strategies that address multiple pollutants rather than separate strategies for each pollutant.

Air quality managers can consider multiple policy goals in developing an air pollution management plan. Policy goals that could be considered in such a program include reducing concentrations of ozone, particulate matter, and hazardous air pollutants (HAPs) such as mercury; planning to address transportation and energy needs; and mitigating and adapting to climate change.

The EPA has encouraged states to take a multi-pollutant approach to managing air quality. North Carolina, New York, and the City of St. Louis (involving both Missouri and Illinois) participated in an EPA-led pilot effort to develop multi-pollutant air quality management plans. The recommended plan would demonstrate the attainment and maintenance of multiple national ambient air quality standards (NAAQS), accomplish sector-based reductions, realize risk reductions of HAPs, and make improvements in visibility.

Frequently asked questions