Short answer: An Extremely Hazardous Substance (EHS) is a chemical listed in Appendices A and B of 40 CFR Part 355. Each has a Threshold Planning Quantity (TPQ). When the total amount of an EHS present at a facility at any one time equals or exceeds its TPQ, the facility must notify the SERC and the LEPC within 60 days and designate a facility emergency coordinator (40 CFR 355.10 and 355.20). Solid EHSs carry two TPQs, and the higher one, 10,000 pounds, applies unless the solid is a fine powder, in solution, molten, or reactive (40 CFR 355.15).
What an EHS and a TPQ are
EPCRA §302 created a list of Extremely Hazardous Substances for local emergency planning. EPA publishes the list with each substance’s TPQ in 40 CFR Part 355: Appendix A in alphabetical order and Appendix B in CAS number order. The TPQ is “the quantity listed in the column ‘threshold planning quantity’” for that substance (40 CFR 355.61). Each entry also lists a reportable quantity (RQ), which is a different number used for release notification under EPCRA §304.
A facility must comply with the §302 emergency planning requirements if either of two conditions is met (40 CFR 355.10):
- Any EHS is present at the facility in an amount equal to or greater than its TPQ.
- The SERC, the Governor, or the chief executive officer of the Tribe has designated the facility for emergency planning, after public notice and opportunity for comment.
Unlike Tier II, §302 is not limited to chemicals that require an OSHA SDS. The test is presence of a listed EHS at or above its TPQ.
Counting toward the TPQ
Add together every pound of the EHS present at the facility at any one time: the pure substance plus the EHS content of every mixture, “regardless of location, number of containers, or method of storage” (40 CFR 355.14). For a mixture, multiply the weight percent of the EHS by the weight of the mixture in each container. An EHS at or below 1% of a mixture does not have to be counted (40 CFR 355.13). When an SDS gives only a concentration range, EPA’s guidance for §302 reporting is to use the upper bound (see mixtures in Tier II).
Selected EHS threshold planning quantities
| Substance | CAS | TPQ (lb) | EHS RQ (lb) | Tier II threshold (lb) |
|---|---|---|---|---|
| Ammonia | 7664-41-7 | 500 | 100 | 500 |
| Chlorine | 7782-50-5 | 100 | 10 | 100 |
| Formaldehyde | 50-00-0 | 500 | 100 | 500 |
| Hydrogen fluoride | 7664-39-3 | 100 | 100 | 100 |
| Hydrogen sulfide | 7783-06-4 | 500 | 100 | 500 |
| Phosgene | 75-44-5 | 10 | 10 | 10 |
| Sulfur dioxide | 7446-09-5 | 500 | 500 | 500 |
| Sulfuric acid | 7664-93-9 | 1,000 | 1,000 | 500 |
Sulfuric acid shows why the two numbers should be kept apart. Its TPQ is 1,000 pounds, but its Tier II threshold is 500 pounds, so a site can owe a Tier II report for sulfuric acid without being subject to §302 for it.
How TPQs for solids work
Some EHSs are listed with two TPQs, written for example as 500/10,000 pounds. These are solids, and 40 CFR 355.15 decides which number applies. Use the lower TPQ if the solid:
- is in powdered form with a particle size less than 100 microns;
- is in solution;
- is in molten form; or
- meets the criteria for an NFPA reactivity rating of 2, 3, or 4.
If none of these applies, the TPQ is 10,000 pounds (40 CFR 355.15(b)). Part 355 calls these “non-reactive solids.” Substances marked with note “a” in the appendix are reactive solids; for them the TPQ does not default to 10,000 pounds in non-powder, non-molten, non-solution form.
For the first three forms, 40 CFR 355.16 also changes how the quantity is counted when you compare it to the lower TPQ:
| Form | Quantity to compare with the lower TPQ |
|---|---|
| Powder, particles under 100 microns | Weight percent of the solid with particle size under 100 microns in the container × total weight of solid in the container |
| Non-reactive solid in solution | Weight percent of the solid in solution × total weight of solution × 0.2 |
| Non-reactive solid, molten | Weight of the molten solid × 0.3 |
Worked example: a solid EHS listed at 1,000/10,000 lb
A site holds a non-reactive solid EHS listed with a TPQ of 1,000/10,000 lb, and its NFPA reactivity rating is below 2.
Flakes in bags, 3,000 lb: not powder, solution, or molten, so the TPQ is 10,000 lb. 3,000 lb is below it.
The same material as a 40% solution, 4,000 lb of solution: the lower TPQ of 1,000 lb applies. Quantity = 0.40 × 4,000 × 0.2 = 320 lb. Below 1,000 lb.
Powder, 2,500 lb, of which 30% by weight is under 100 microns: quantity = 0.30 × 2,500 = 750 lb. Below 1,000 lb.
For Tier II the 0.2 factor does not apply. The solution holds 0.40 × 4,000 = 1,600 lb of the EHS, and the Tier II threshold is the lower of 500 lb and the TPQ, so the EHS is reportable on Tier II even though no §302 notice is due. Whether the separate forms must be combined into one total for the TPQ test is not spelled out in 355.15–355.16; if they could together reach a TPQ, ask your SERC or EPA.
The 60-day notice and the facility emergency coordinator
| Requirement | To whom | When |
|---|---|---|
| Notice that the facility is subject to emergency planning | SERC and LEPC | Within 60 days after first becoming subject |
| Designate a facility emergency coordinator and give notice of who it is | LEPC (or SERC if no LEPC, or the Governor if no SERC) | Within 60 days after first becoming subject |
| Changes at the facility relevant to emergency planning | LEPC | Within 30 days after the change |
| Information the LEPC requests to develop or carry out the local plan | LEPC | Promptly; the LEPC may set a time frame |
EPA does not require a specific format and recommends submitting the information in writing; the SERC or LEPC may ask for a particular format (40 CFR 355.21). The facility emergency coordinator takes part in the local emergency planning process. The Tier II form asks whether the facility is subject to §302 and for the coordinator’s name, title, phone, 24-hour phone, and email (40 CFR 370.42(k) and (o)).
How the TPQ relates to Tier II
The TPQ shows up in two places. Under §302 it is the planning threshold itself. Under Tier II, the reporting threshold for an EHS is 500 pounds or the TPQ, whichever is lower (40 CFR 370.10(a)(1)). So:
- An EHS with a TPQ of 500 pounds or less has the same number for both programs.
- An EHS with a TPQ above 500 pounds, such as sulfuric acid at 1,000, becomes reportable on Tier II at 500 pounds, before §302 is triggered.
- The solution and molten reductions in 355.16 apply to the TPQ test only. Tier II uses the actual amount present.
§302 is a one-time notice when the facility first becomes subject. Tier II repeats every year. See Tier II reporting for the annual report and thresholds compared for how the TPQ sits next to RMP, PSM, and CERCLA quantities.
Common questions
What is a threshold planning quantity?
The quantity listed for each Extremely Hazardous Substance in 40 CFR Part 355, Appendices A and B. If the total amount of that EHS present at a facility at any one time equals or exceeds its TPQ, the facility is subject to EPCRA section 302 emergency planning requirements.
What is the deadline for EPCRA 302 notification?
Within 60 days after the facility first becomes subject, notify the SERC and the LEPC, and designate a facility emergency coordinator and notify the LEPC of that person (40 CFR 355.20). Changes relevant to emergency planning must be reported to the LEPC within 30 days.
Why do some EHSs have two TPQs, like 500/10,000?
They are solids. The lower TPQ applies if the solid is a powder with particles under 100 microns, in solution, molten, or has an NFPA reactivity rating of 2, 3, or 4. Otherwise the TPQ is 10,000 pounds (40 CFR 355.15).
Is the Tier II EHS threshold the same as the TPQ?
Not always. The Tier II threshold for an EHS is 500 pounds or the TPQ, whichever is lower (40 CFR 370.10). For an EHS with a TPQ above 500 pounds, the Tier II threshold is 500 pounds.
Do I count an EHS that is in a mixture?
Yes. Multiply the EHS weight percent by the weight of the mixture and add it to all other quantities of that EHS at the facility. An EHS at or below 1% of a mixture does not have to be counted (40 CFR 355.13 and 355.14).
Where Chemsana fits
Chemsana checks each chemical approval and receipt against the EPCRA 302 EHS list and TPQs, totals the EHS by CAS number across products using the upper bound of SDS ranges, and returns an Allowed, Flag, or Block decision with the citation and the math. Release screening compares spill amounts against CERCLA and EHS RQs. See intake decisions.
Sources
- 40 CFR Part 355, Emergency Planning and Notification (§§355.10–355.21, 355.61, Appendices A and B), eCFR, accessed October 2026.
- 40 CFR 370.10, thresholds for hazardous chemical reporting, eCFR, accessed October 2026.
- 40 CFR 370.42, Tier II inventory information, eCFR, accessed October 2026.
- Reporting for an EHS mixture based on a concentration range, EPA frequent question, accessed October 2026.
This guide summarizes federal requirements for general information. States can set lower thresholds or extra requirements, and it is not legal advice. Confirm obligations with your SERC, LEPC, or a qualified EHS professional.