Ontario Workplace Chemical Exposure Limits
How do Ontario's workplace chemical exposure limits work?
Ontario's workplace chemical exposure limits come from Regulation 833 (Control of Exposure to Biological or Chemical Agents) under the Occupational Health and Safety Act. The rule has two tiers. If an agent is in the Ontario Table (Table 1 of the Regulation), that limit applies. If it isn't, the employer "shall not exceed" the TWA, STEL or ceiling in the ACGIH Table [1]. The ACGIH Table is fixed to one edition: the "Adopted Values" in the 2017 ACGIH TLVs and BEIs publication [2]. Later ACGIH updates do not apply automatically.
Approach: I read the current consolidated Regulation 833 (Ontario e-Laws, ontario.ca/laws/regulation/900833) and the Ministry's OEL and designated-substances guidance pages. I did not read the text of O. Reg. 490/09 (Designated Substances) itself.
1. Which limit applies (s. 4)
- Agent in the Ontario Table: the Ontario TWA, STEL or C applies.
- Not in the Ontario Table but in the 2017 ACGIH Table: the ACGIH value applies.
- Excursion limits: if the table gives only a TWA, with no STEL or C, exposure shall not exceed 3× the TWA for any 30-minute period and 5× the TWA at any time. This does not apply to designated substances under O. Reg. 490/09 [1].
The Ministry publishes a combined Ontario/ACGIH table "for convenience." It also says the table "should not be solely relied upon," because the Regulation holds the full detail, such as the excursion limits. The page was last updated 26 June 2025. A 2021 consultation on new and revised limits is still "under consideration" (Ministry page).
2. Definitions (s. 1)
- TWA: the time-weighted average concentration a worker may be exposed to over a work day or work week.
- STEL: the maximum for any 15-minute period, no more than four times per 8-hour shift, with at least one hour between exposures.
- Ceiling (C): the maximum concentration at any time.
- "Exposure" covers inhalation, ingestion, skin absorption and skin contact.
3. The Ontario Table (Table 1)
The table gives Ontario-specific values, in ppm, mg/m³ or f/cc, with notations. Some examples:
- Benzene: 0.5 ppm TWA, 2.5 ppm STEL, Skin
- Formaldehyde: STEL 1 ppm or C 1.5 ppm
- Hydrogen sulfide: 10 ppm TWA, 15 ppm STEL
- Crystalline silica (quartz): 0.10 mg/m³ (respirable)
- Isocyanates such as TDI, MDI and HDI: 0.005 ppm TWA, C 0.02 ppm
Notations used in the table:
- (I): inhalable fraction.
- (R): respirable fraction.
- (F): fibre counting method.
- (L): "Exposure by all routes should be carefully controlled to levels as low as possible."
Additional duties tied to notations:
- "Skin" notation: the employer must take all reasonably necessary measures to prevent skin absorption (s. 7) [8].
- ACGIH "Simple asphyxiant" or "Minimal Oxygen Content" entries: the employer must protect workers from oxygen below 19.5% by volume and from related fire and explosion hazards (s. 7.1) [9].
4. Designated substances are carved out (s. 2(2))
Regulation 833 does not apply to:
- a Table 1 agent of O. Reg. 490/09 (Designated Substances), in a workplace subject to that regulation for that agent;
- asbestos, in workplaces covered by O. Reg. 278/05.
The Ontario Table marks these agents with an asterisk [6]. According to the Ministry guide, the 11 designated substances are acrylonitrile, arsenic, asbestos, benzene, coke oven emissions, ethylene oxide, isocyanates, lead, mercury, silica and vinyl chloride. O. Reg. 490/09 requires a written assessment done in consultation with the joint health and safety committee (JHSC), and a control program where exposure is likely. That program covers monitoring, records, medical examinations for some substances, and training. The guide says construction projects and farming operations are excluded (Ministry guide).
5. Measuring and calculating exposure (s. 6, Schedule 1)
- Sampling method: sampling must follow a standard method from ASTM, HSE (UK), IRSST, ISO, NIOSH, OSHA or US EPA, or another method recognised in industrial hygiene practice. It must be done by, or under the direction of, a person qualified in industrial hygiene. Direct-reading instruments must be used, calibrated and maintained to the manufacturer's instructions [11][3].
- Respirators are ignored when determining exposure.
- Sampling time: sampling need not cover the full shift if it is representative.
- TWA calculation: add up concentration × hours (ΣCᵢTᵢ), then divide by 8 for a daily TWA or by 40 for a weekly TWA.
- Extended shifts: the employer "may" adjust the TWA using the IRSST Guide for the Adjustment of Permissible Exposure Values for Unusual Work Schedules (March 2015).
- STEL measurement: a single 15-minute sample, or a time-weighted average of sequential samples within that period.
- Mixtures with additive effects: Σ(Cᵢ/Lᵢ) shall not exceed 1. Agents with substantially different health effects are assessed separately.
6. Control obligations behind the limits
- General duty: employers "shall take all measures reasonably necessary" to protect workers. These include substitution, engineering controls, administrative controls and hygiene, with PPE only where s. 7.2 applies (s. 3) [13].
- Respirators are a last resort: they may be required only where substitution is not reasonable or practical, and engineering controls do not exist, are impractical, have temporarily broken down, or fail in an emergency (s. 7.2) [14].
- Respiratory protection program: this applies once respirators are provided (ss. 9–13) [15][16][17]. It requires:
- written selection and use procedures and training;
- NIOSH- or CSA-approved respirators (or an equivalent judged by a qualified person) that meet the Schedule 2 assigned protection factors;
- fit testing under CSA Z94.4-18, and no facial hair under tight-fitting seals.
- Respirator selection: the "maximum use concentration" is the respirator's assigned protection factor multiplied by the Ontario or ACGIH OEL [2].
- Medical examinations (s. 8): if exposure is believed to have affected, or to be likely to affect, a worker's health, the worker, if they agree, undergoes examinations and tests at the employer's expense [18].
The consolidated text includes amendments up to O. Reg. 113/26, which changed s. 1 and s. 10 [3][16].
I can look up the applicable Ontario limit for specific substances on request.
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