At the root of air quality regulations, there is a fundamental question: Which is more important, strict enforcement of concentration limits in the permits, or overall protection against health risks?

The criteria pollutants, as defined by the U.S. National Ambient Air Quality Standards for harmful pollutants, include carbon monoxide (CO), lead, nitrogen oxides (NOX), particulate matter (PM), sulfur oxides (SO2) and volatile organic compounds (VOCs). These standards were established by the U.S. Environmental Protection Agency under authority of the Clean Air Act (42 U.S.C. 7401 et seq.). 

In a separate class, there are a large number of toxic compounds that agencies in California and nationally have regulated because of their hazardous properties (e.g., carcinogenic or toxicity). Those compounds became a focus for regulatory bodies beginning around 1990 and are increasingly a subject of active monitoring and public data sharing.

The threshold used to establish limits for these toxic compounds is defined as a hazard index (HI). A hazard index of 1 or lower means adverse noncancer effects over a lifetime of exposure are unlikely, and thus can be considered to have negligible hazard. For a value greater than 1, the potential for adverse effects increases.

Because different air toxics can cause similar adverse health effects, combining hazard quotients from different toxics is often appropriate. The regulatory agencies evaluate this on a case-by-case basis. Typically, any emission source that has a potential to emit toxic air pollutants beyond the thresholds (for example, an HI of 1) established by the respective regulatory agency will need an air permit and may be subject to stringent emission control, risk reduction measures and monitoring requirements.

Typically, air quality permits delineate specific toxic compounds and their individual concentration limits. Under standard permit verbiage, if even a single toxic compound exceeds its limit, the operation is viewed as noncompliant and must shut down until it can be remedied.

Concentration limits are part of the permit because they make operating conditions easy to understand and enforce, but they are essentially derived from the overall hazard index threshold calculated for the specific permitted unit.

Thinking along the same lines, if one of a dozen regulated toxic compounds temporarily fluctuates above its concentration limit while the rest are significantly below the limits, does that really pose a significant enough risk, when viewed as part of a bigger picture, to merit the loss of operation and money? Does that truly benefit the public or make a significant difference in health impacts, when other compounds are well below limits?

What if there were a different, more practical way to look at a scenario’s potential toxic risk? If health is the bottom line, attention properly belongs on how concentrations are affecting the public, rather than a strict quantitative focus on permit limits. That’s why we work with local permitting districts to obtain permits that help owners operate more flexibly while remaining in compliance with the rules.


This can be achieved by incorporating a toxic risk assessment condition in the permit language that states: if the collective allowable risk limit is not exceeded, the unit may continue operation, and compliance with health hazard limits should be demonstrated by running a screening health risk assessment for observed concentration levels. This strategic approach aims to protect both public health and facility operational status by recognizing that minor daily fluctuations in the concentration of a few compounds may be less significant when the other compounds are far below their limits. In that everyday scenario, the overall calculated risk might be well below the threshold.

Having individual concentration limits provides a comforting fallback position and an easily enforceable permit condition. However, a reasonable, strategic compromise in permitting language offers the flexibility to keep businesses open while retaining the primary objective of protecting public safety.


Controlling air emissions involves understanding how they disperse. Thus, determining the right height for an exhaust stack is a blend of art and science; air dispersion modeling helps you include all relevant factors into your calculations.

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