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Air Toxics Screening Levels and How They Affect the Air Permit Application Process

Did you ever wonder why it seems like it takes so much time getting your air permit back from the state? Cheer up as it’s become much more efficient in recent years, especially if you remember back to the 1990s when a permit might take up to two years to issue. Today it’s a different situation. Michigan’s AQD rule 206 requires:

“The department shall review an application for a permit to install for administrative completeness pursuant to R 336.1203(1) within 10 days of its receipt by the department. The department shall notify the applicant in writing regarding the receipt and completeness of the application.”


“The department shall take final action to approve or deny a permit within 180 days of receipt of an application for a permit to install. The department shall take final action to approve or deny a permit to install subject to a public comment period pursuant to R 336.1205(1)(b) or section 5511(3) of the act within 240 days of receipt.”

Except for ROP renewals, not many permits get to public comment, so 180 days is standard. But very few applications take this long to review, process, and receive issuance. In fact, very few permits are even denied. There are times, however, when companies need this process to go faster. Certainly, finding a permit amendment to begin installing is one option and a waiver to commence construction is another. But neither of these options might be necessary if the permit would just come back a little bit quicker.

So, what’s the holdup? In most (nearly all) cases, the limiting time constraint is wading through the air toxics rules, also known as T-BACT (Toxics Best Available Control Technology). What is T-BACT? In a nutshell, the specific compounds proposed to be emitted cannot exceed the maximum allowable emission rate which results in a predicted maximum ambient impact that is more than the initial threshold screening level or the initial risk screening level, or both. Putting this into English, AQD has a list of “screening levels” for the community. I look at them as OSHA-like exposure limits for the community. A computer program is used to predict the ground level concentration of these specific compounds. If the predicted concentration exceeds the screening level concentration, then an emission limit on the specific compound is incorporated into your permit.

Sometimes these emission limits are annual (lbs/yr), other times the limit is daily (lbs/day), and sometimes even on a per shift basis (lbs/8-hours) or hourly (lbs/hr). At a minimum, prepare for extra record keeping to demonstrate compliance. At other times, process or stack changes will be required.

You’ve received your permit and can show compliance with all the air toxics limits in your permit. Let’s fast forward a year or two. You want to use different materials in your process. There’s now a new list of air toxics and you’re able to show that the toxicity of the new compounds does not exceed the toxicity of the original compounds (a whole other blog topic). Your good to go ahead and change materials.

Fast forward another year or two. Now you’re approaching your VOC emission limit and require an amendment. The permit process should be smoother this time as the hiccups from the original review have been resolved. Turns out, you’re wrong as the screening level for one of the compounds has changed and is now lower. If you didn’t ask for a permit amendment, the screening level in effect during the original permit review is still what it was in the first place. The new review has the affect of updating the screening levels. Now the process that was fine to operate yesterday may not be fine to operate tomorrow.

Note that while none of the rules have changed, the criteria for approving your permit application definitely has changed. The toxicologists at AQD are continuously revisiting screening levels. In fact, in May 2019, the AQD took another look at isopropyl alcohol. I’d like to believe that we know everything there is to know about isopropyl alcohol, but apparently not. In this case, one of the screening levels was left as is and another was removed. This is not always the case. We have a customer who uses a degreaser that contains “dibasic esters” that are considered by the manufacturer to be environmentally friendly. In fact, the supplier’s SDS indicates that the formulation is “Accepted In Partnership With The Design for the Environment Division Of The EPA.” AQD has a screening level for these compounds of 1 ug/m3. That’s microgram per cubic meter… 3,000 times less than the OSHA TWA for benzene (you read that right).

If there’s a lesson here, it’s to understand the permit review process and the inevitable conclusions prior to submitting your application and to prepare to address them up front. CFR has been preparing permit applications and helping navigate these waters for over 30 years. Contact CFR to learn how we can assist.

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