Visible Emissions – What You Need to Know

If you have an air permit for your operations, it likely includes limits on visible emissions regardless of the state in which you do business, the type of air permit, or the type of business.  Here’s what you need to know about what visible emissions are, how they’re regulated, and how to stay in compliance with your air permit.

What are visible emissions?  Visible emissions are exactly what you would expect – emissions that can be seen by the human eye. They may be vented through stacks or come from fugitive sources (e.g. boilers fueled by a solid fuel or diesel oil, aggregate crushers, metal shredders, welding booths, gravel piles, dirt roads).

People often use the term “smoke” to describe visible emissions because incomplete combustion of solid and liquid fuels like coal and diesel oil in boilers, furnaces, generators and other combustion equipment were the focus of the original visible emissions evaluation system developed in the late 1800’s. Since then, evaluation of visible emissions has expanded to include non-combustion equipment because visible emissions result from particulate matter emitted by many types of equipment or operations.  While not always generated from combustion, the term “smoke” is still widely used to describe visible emissions.

What is opacity and why is it important?  Opacity is a measure of how much light is blocked by the visible emissions plume.  High opacity means dirtier smoke, or a higher concentration of particulate matter in the exhaust air emitted from a stack or a fugitive source. 

How are visible emissions regulated?  Federal opacity standards for various industries are found in 40 CFR Part 60 (Standards of Performance for New and Modified Stationary Sources), and 40 CFR Parts 61 and 62 (Emission Standards for Hazardous Air Pollutants).  These standards require the use of EPA Reference Method 9 or Reference Method 22 for the determination of the level (opacity) or frequency of visible emissions by trained observers.  Reference Methods are found in Appendix A of 40 CFR Part 60.  State agencies may adopt the federal opacity standards; however, some states adopt more stringent standards. The allowable visible emissions standards often vary by industry.

What requirements may be included in an air permit?  An air permit will typically limit the maximum allowable opacity across a specified time period from each source of particulate matter emissions.  For example, a permit condition may read “visible particulate emissions from any stack shall not exceed 20 percent opacity, as a 6-minute average, except for a period of 6 consecutive minutes in any 60 minutes.”

How can you demonstrate compliance with a visible emissions limit?  Your air permit will not only include a limit on visible emissions measured in terms of opacity, and sometimes the frequency, but it will also include the appropriate reference test method that you must use to demonstrate compliance when testing is required. EPA Method 9 is the primary test method for visible emissions and is applied to emissions from stacks.  If visible emissions testing is required and the demonstration of compliance is by EPA Method 9, a certified visible emissions reader must perform the test. Typically, environmental agencies require a test plan and advanced notice of the test when Method 9 is being used to demonstrate compliance. The results of the Method 9 test must be summarized in a written report and submitted to the regulatory agency.

EPA Method 22 is a test method for demonstrating compliance from fugitive sources and from flares. It is not an alternative method to EPA Method 9. Instead of determining the opacity of emissions, EPA Method 22 determines the frequency of visible emissions.  Unlike EPA Method 9, there is no certification requirement for the person conducting EPA Method 22 testing. However, it is strongly recommended that you are familiar with visible emissions observations by completing a lecture course for EPA Method 9 and at least one “smoke” school test offered by the state environmental agency or a private firm. 

What else do you need to know about visible emissions?  Some agencies consider the permitted source’s potential for generating visible emissions under normal operating conditions and only require an initial test upon startup of the new equipment. If the emissions source is controlled, then a visible emissions test may not be required at all.  For example, a metal or plastic parts coating spray booth controlled by overspray filters may have a permit condition to operate and maintain the filters in place of conducting periodic EPA Method 9 testing. Keep in mind that the visible emissions requirement applies during periods of startup, shut down, and malfunction when the control device may not be operational. It is recommended that the source have a record of the opacity and/or frequency of visible emissions during these situations.

By understanding these definitions and following the guidelines, you’ll remain in compliance with your air permit and successfully meet your visible emissions requirements.


Ann O’Brien, SCS Engineers