Air contaminants exposure limits

A limit preceded by "C" is a ceiling and the exposure must never exceed that value; if you cannot do instantaneous monitoring, assess the ceiling as a 15‑minute time‑weighted average that must not be exceeded at any time during the workday.

• The rule is in 1910.1000(a)(1).
• Practical tip: when you cannot get continuous or instantaneous samples, collect 15‑minute samples and ensure none exceed the ceiling value.

You must ensure the employee's exposure does not exceed the 8‑hour Time Weighted Average (TWA) listed for that substance in any 8‑hour work shift of a 40‑hour work week.

• Requirement text: 1910.1000(a)(2).
• Practical tip: calculate an 8‑hour TWA from sampled concentrations and durations to confirm compliance.

For Table Z‑2 substances you must keep exposures below the 8‑hour TWA, not exceed the acceptable ceiling at any time except as allowed by the table, and any short peaks above the ceiling are limited to the maximum duration and concentration shown.

• See 1910.1000(b)(1) and 1910.1000(b)(2).
• Example from the standard: an employee with a 10 ppm TWA, 25 ppm ceiling, and 50 ppm peak may be above 25 ppm (but never above 50 ppm) only for the maximum minutes allowed and only if the overall 8‑hour weighted exposure remains at or below 10 ppm; see 1910.1000(b)(3).

Compute E using E = (Ca Ta + Cb Tb + ... + Cn Tn) ÷ 8 and ensure E does not exceed the 8‑hour TWA PEL for the substance.

• See the formula in 1910.1000(d)(1)(i).
• Practical steps: multiply each constant concentration C by its duration T in hours, add the products, divide by 8, and compare the result to the listed 8‑hour PEL.

Yes — applying the formula gives E = (2×150 + 2×75 + 4×50) ÷ 8 = 81.25 ppm, which is less than the 100 ppm 8‑hour PEL, so the exposure is acceptable.

• This illustrative calculation appears in 1910.1000(d)(1)(ii).
• Practical tip: always round and document the sampling intervals and concentrations used in the calculation.

Calculate Em = (C1 ÷ L1) + (C2 ÷ L2) + ... + (Cn ÷ Ln) for the mixture and ensure Em does not exceed unity (1); if Em > 1, the exposure exceeds the allowable limit.

• See the mixture formula in 1910.1000(d)(2)(i).
• Practical tip: C is the measured concentration for each contaminant and L is that contaminant's 8‑hour PEL from subpart Z.

Yes — using the example: Em = 500 ÷ 1000 + 45 ÷ 200 + 40 ÷ 200 = 0.500 + 0.225 + 0.200 = 0.925; because Em is less than 1, the combined exposure is within acceptable limits.

• See this illustration in 1910.1000(d)(2)(ii).
• Practical tip: compute each fraction to at least three decimal places for accuracy, then sum and compare to 1.

Employers must first determine and implement feasible engineering or administrative controls to achieve compliance with the exposure limits in paragraphs (a) through (d).

• See the requirement in 1910.1000(e).
• Practical examples: local exhaust ventilation, process modification, substitution, scheduling changes, and reduced shift durations before relying on personal protective equipment.

Respirators may be used only when engineering and administrative controls are not feasible to achieve full compliance, and any respirator use must comply with 1910.134.

• See the respirator sentence in 1910.1000(e) and the respiratory protection standard at 1910.134.
• Practical tip: if you rely on respirators, implement a written respiratory protection program, fit‑testing, medical evaluation, and training as required by 1910.134.

Any equipment or technical measures used to keep exposures within limits must be approved for each particular use by a competent industrial hygienist or other technically qualified person.

• See this requirement in 1910.1000(e).
• Practical tip: document the qualified reviewer’s credentials and their written approval when relying on such measures.

Yes — short exposures above the Table Z‑2 acceptable ceiling are allowed only up to the maximum peak concentration and duration shown in the table, and such peaks must be compensated by lower exposures so the overall 8‑hour TWA stays at or below the listed limit.

• See the example and rule in 1910.1000(b)(2) and 1910.1000(b)(3).
• Practical tip: when peaks occur, record their duration and concentration and recompute the 8‑hour weighted average to document compliance.

Substances in Table Z‑1 are subject to either ceiling (marked with "C") or an 8‑hour TWA (if not marked "C"); Table Z‑2 entries have 8‑hour TWAs plus specified acceptable ceilings and allowable peaks; Table Z‑3 entries must not exceed their listed 8‑hour TWA.

• See 1910.1000(a) for Table Z‑1, 1910.1000(b) for Table Z‑2, and 1910.1000(c) for Table Z‑3.
• Practical tip: always consult the specific table entries because the numeric limits and peak rules differ by substance.

You must compute the cumulative exposure across the entire 8‑hour work shift using the E formula [(Ca Ta + ... ) ÷ 8] so the single 8‑hour equivalent E does not exceed the 8‑hour PEL.

• See 1910.1000(d)(1)(i).
• Practical tip: break the shift into periods where concentration is reasonably constant, document durations, and include all periods in the calculation.

If Em exceeds 1, the combined exposures are above allowable limits and you must implement feasible engineering or administrative controls to reduce exposures; respirators or other protective measures can be used only when controls cannot fully reduce exposure.

• See the mixture criterion in 1910.1000(d)(2)(i) and the control-first requirement in 1910.1000(e).
• Practical tip: prioritize source enclosure, ventilation, or process change; document why any remaining exposures still require PPE and have a qualified person approve the measures.

An oxygen‑deficient atmosphere (below 19.5% oxygen per 1910.134) is considered IDLH and affects respirator choice and program requirements; OSHA has discussed this issue in its letter about oxygen‑deficient atmospheres in HVAC contexts.

• See the respiratory protection rule at 1910.134 and OSHA’s interpretation at https://www.osha.gov/laws-regs/standardinterpretations/2024-07-16 which addresses oxygen‑deficient atmospheres and applicability.
• Practical tip: when oxygen is deficient, use supplied‑air or SCBA respirators as required by [1910.134] and document the hazard assessment and medical evaluations.

No — employers must first implement feasible engineering or administrative controls, and respirators may be used only when those controls are not feasible to achieve full compliance.

• See the control hierarchy requirement in 1910.1000(e).
• Practical tip: treat respirators as an interim or supplemental measure and maintain documentation showing why controls could not achieve the PEL before relying on respirators.