Why we don't say 'compliant' or 'non-compliant'
A compliance verdict on short-term exposure requires far more context than any calculator can hold. Rules such as the STEL limit or the 3×/5× excursion rule are necessary conditions — but not sufficient ones.
A real compliance determination also requires: the specific sampling strategy used (grab sample vs. direct-reading instrument vs. personal dosimeter), the measurement uncertainty associated with the analytical method, whether the substance is a sensitizer or exhibits skin absorption, the presence of mixture effects (ACGIH additive formula), the population of workers (age, pregnancy, pre-existing conditions), and the applicable regulatory reference for the specific jurisdiction and substance.
This tool gives you the factual rule-by-rule diagnostic. The interpretation — is this acceptable for this worker, in this context, with this measurement quality? — belongs to a certified industrial hygienist.
STEL vs Ceiling vs Excursion limit — the differences
These three concepts are often confused but represent distinct constraints:
- STEL / VECD (Short-Term Exposure Limit / Valeur d'exposition de courte durée): A 15-minute time-weighted average that must not be exceeded, with a maximum of 4 such excursions per day and at least 60 minutes between consecutive excursions. It is designed to protect against acute effects distinct from chronic effects covered by the TWA. Not all substances have a STEL — when absent, the default excursion rule applies.
- Ceiling / Plafond (TLV-C / Valeur plafond): A concentration that must never be exceeded, even instantaneously. Unlike STEL, there is no averaging period — it applies moment to moment. Ceilings are used for substances with such severe acute hazards (e.g., strong irritants, asphyxiants) that any exceedance is unacceptable. A reading above the ceiling at any moment is a violation regardless of duration.
- ACGIH default excursion rule (when no STEL exists): When a substance has a TLV-TWA but no TLV-STEL, ACGIH allows excursions up to 3× the TLV-TWA provided the total time above 3× does not exceed 30 minutes per day, and 5× the TLV-TWA must never be exceeded even briefly. This is sometimes called the "excursion limit" and provides a backstop for substances without published STELs.
In Quebec (RSST), the equivalent terms are VEMP (8-hour), VECD (15-min), and valeur plafond (ceiling). The RSST Annex I lists these values for regulated substances. When the RSST VECD is absent, the ACGIH excursion rule is sometimes used as a supplementary reference, but this must be confirmed with the applicable regulatory authority.
Frequently Asked Questions
What is a STEL?
A STEL (Short-Term Exposure Limit) is a 15-minute time-weighted average concentration that must not be exceeded at any point during a workday. It is specifically designed to protect against acute effects — irritation, narcosis, sensory threshold effects — that can occur over short exposures even when the 8-hour TWA remains below its limit. ACGIH sets TLV-STELs for substances with documented acute hazards at sub-TWA levels. In Quebec (RSST), the equivalent is the VECD (Valeur d'exposition de courte durée). Not all substances have a STEL; for those that do not, the ACGIH default excursion rule applies.
Does the 3×/5× rule apply to all substances?
No. The ACGIH excursion rule (3×/5× TLV-TWA) applies only to substances that have a TLV-TWA but no published TLV-STEL. If a substance has a TLV-STEL, use the STEL directly — the 3×/5× rule is not a substitute or supplement for a STEL. Additionally, ACGIH notes that the 3×/5× rule does not apply to substances that are primarily carcinogens, reproductive hazards, or sensitizers — for those, any excursion above the TLV-TWA may be of concern regardless of the rule. Always verify whether a STEL exists for the specific substance before defaulting to the excursion rule.
What if my peak is 14 minutes and 59 seconds?
Technically, a 15-minute STEL is a 15-minute time-weighted average — so a peak ending at 14:59 is compliant with the duration rule. However, industrial hygiene sampling rarely provides sub-minute resolution, and a peak this close to the 15-minute boundary warrants scrutiny. If the measurement method is a direct-reading instrument, the exact cutoff matters. If it is a grab sample collected over a defined period, confirm the actual sampling duration. In practice, if your peak is repeatedly hitting 14–15 minutes, the margin is too thin for comfort — consider engineering controls or administrative limitations that keep the peak well below 15 minutes.
How does this differ between Quebec, US-Federal (OSHA), and ACGIH?
Quebec (RSST): The VECD is a legally binding limit in Quebec under the RSST Regulation. The Annex I of the RSST lists VEMP (TWA), VECD (STEL), and valeurs plafond for regulated substances. These are mandatory for Quebec employers. OSHA (US Federal): OSHA STEL values appear in specific substance standards (e.g., 1910.1001 for asbestos) but are not systematically established across all substances in Table Z-1. Many OSHA limits are based on ACGIH 1968 values and have not been updated. For substances without OSHA STELs, NIOSH RELs or ACGIH TLVs are often used as supplementary references. ACGIH: TLV-STELs are published annually in the TLV/BEI Booklet and are updated as evidence accumulates. They are guidelines — not legal requirements in themselves — but are widely adopted as the scientific basis for regulatory limits worldwide.
Should the 60-minute spacing rule apply if I only have one or two peaks?
The 60-minute spacing requirement between excursions only becomes relevant when you have two or more peaks in a single workday, and both peaks have documented start times. With a single peak, there is no predecessor to measure spacing against. With two peaks but no start times, this tool notes the rule as unverifiable and skips it. If you do have consecutive peaks with known times, enter the start time for each and the checker will calculate the gap between the end of the previous peak and the start of the next. Note that the 60-minute rule is an ACGIH requirement specifically referenced in the context of STEL/Mode A; when using Mode B (3×/5× excursion rule), no spacing requirement is specified.
When does the 3× / 5× excursion rule actually apply?
The ACGIH default excursion rule (peak ≤ 3× TWA total time / never > 5× TWA) is meant ONLY for substances that lack a published STEL or Ceiling in the TLV Booklet. If a substance has a STEL, the STEL rules apply (15 min average, max 4/day, 60 min spacing) and the 3×/5× rule does not. Some substances also have substance-specific excursion guidance — check the TLV documentation. Quebec RSST and OSHA do not have an equivalent default excursion rule, only the substances explicitly listed.
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