Protecting Public Health and the Environment.

Visible Emissions

What Are Visible Emissions?

Visible emissions are comprised of a variety of particulate matter ranging in sizes from 0.1 micrometer to 200 micrometers in diameter (compared to the average human hair, which is 70 micrometers in diameter).

Particles are categorized as follows:

  • Smoke
  • Soot
  • Dust
  • Fly ash
  • Fumes
  • Liquid droplets

Particles also are formed in the atmosphere by condensation or transformation of emitted gases, such as sulfur dioxide, nitrogen oxides, and volatile organic compounds, into tiny droplets.

What Are the Impacts of Visible Emissions?

Inhaled particles can be hazardous to your health. Smoke and dust are the most common types of visible emissions. They are comprised of inhalable particulate matter made up of microscopic solid or liquid particles. Major human health concerns include effects on breathing and respiratory functions, aggravation of existing respiratory and cardiovascular disease, alterations in the body's defense systems against foreign materials, damage to lung tissue, carcinogenesis, and premature death.

Particulate matter also may injure crops, trees, and shrubs and may damage metal surfaces and fabrics. Fine particles also impair visibility by scattering light and reducing the visibility.

How Are Visible Emissions Measured?

The Ringelmann Chart was developed in the late 1800s and became one of the first tools used to measure visible emissions. Introduced into the United States in 1897, it was soon accepted as the standard measure of black smoke density and later adapted for gray, white, and other colors of smoke plumes. This then became the basis for many city, state, and federal regulations on smoke density. The Ringelmann Chart is based on the premise that the darker the plume, the more particles are present to block the light and reduce visibility.

Plume opacity is measured in percent: the greater the opacity, the more the background behind the plume is obscured and the less light can come through the plume. If none of the background is obscured, then the opacity is 0%. If the entire background is obscured, then the opacity is 100%.

EPA Reference Method 9

EPA Reference Method 9 is found in 40 CFR 60, Appendix A. It was adopted as a visible emissions inspection method in an effort to standardize the training and certification of observers and to ensure that reliable and repeatable opacity observations could be conducted anywhere in the United States.

Idaho Visible Emissions Limit

Idaho's methods for determining opacity are found in DEQ's Procedures Manual for Air Pollution Control (1986). These procedures are nearly identical to those contained in EPA Reference Method 9. Both describe the requirements for training and testing opacity observers, steps to follow, and data to record while documenting an observation.

The major difference between Idaho's method for making opacity determinations and Reference Method 9 is how opacity exceedances are calculated. The Rules for Control of Air Pollution in Idaho (IDAPA limit visible emissions from any point of emission to 20% opacity for more than 3 minutes in any 60-minute period.

(Different standards apply to six exempted sources. See the rules for more information about these sources. Additionally, more stringent visible emissions standards may apply to sources subject to Federal New Source Performance Standards found in 40 CFR 60.)

How Are Visible Emissions Violations Measured in Idaho?

Businesses that generate visible emissions can follow these steps to measure compliance with Idaho's visible emissions standard:

Step 1: Count the number of readings in excess of the percent opacity limitation (in most cases, 20%).

Step 2: Divide this number by four (each reading represents 15 seconds) to find the number of minutes in excess of the percent opacity limitation.

Step 3: If the opacity limit has not been exceeded for more than 3 minutes, no violation has occurred.

Because Reference Method 9 calculates opacity differently than the Idaho rule, sources subject to federal New Source Performance Standards must calculate opacity as described above and as specified in Reference Method 9.

According to Reference Method 9, a violation has occurred if the average of any group of 24 consecutive readings (6 minutes) in a 1-hour period exceeds the standard.

Consequences of Noncompliance

Failure to comply with the visible emissions standard may result in enforcement action by DEQ with possible penalties assessed.


Note: This information summarizes Idaho's visible emissions requirements and is not intended to be all-inclusive. Detailed requirements are outlined in the Rules for the Control of Air Pollution in Idaho (IDAPA

Staff Contacts

Stationary Source Program Manager
Michael Simon
DEQ State Office
Air Quality Division
1410 N. Hilton
Boise, ID 83706
(208) 373-0212

DEQ Resources