O Reg 833 and Ontario workplace exposure limits explained
O Reg 833 sets Ontario workplace chemical exposure duties. Use Table 1 first. Use 2017 ACGIH limits only when the ACGIH table lists the agent.
O Reg 833 is Ontario's R.R.O. 1990, Regulation 833, Control of Exposure to Biological or Chemical Agents, under the Occupational Health and Safety Act. Use its Ontario Table first, and use the incorporated 2017 ACGIH limits only when the agent is absent from that table but listed in the ACGIH table.
Finding a number is only part of the task. Employers must also check which regulation applies and assess exposure. Controls must address the relevant exposure routes, including skin contact.
Check whether Regulation 833 applies
Section 3 requires all measures reasonably necessary to protect workers from hazardous biological or chemical agents. This covers agents arising from storage, handling, processing or use. The duty is not limited to chemicals with a numerical occupational exposure limit. Exposure includes inhalation, ingestion, skin absorption and skin contact. See sections 1 and 3.
Start with the substance and the work being done. Section 2 excludes a designated substance where the workplace is subject to O. Reg. 490/09 for that agent. It separately excludes asbestos where the workplace is subject to O. Reg. 278/05. These are conditional exclusions, not blanket exemptions for every task involving those substances. Check section 2 before choosing a limit.
O. Reg. 490/09 covers 11 designated substances: acrylonitrile, arsenic, asbestos, benzene, coke oven emissions, ethylene oxide, isocyanates, lead, mercury, silica and vinyl chloride. Its substance-specific provisions determine coverage. Its construction exclusion, in section 14, does not mean construction work is automatically outside Regulation 833.
How to use O Reg 833 Table 1
The legal sequence in section 4 is:
- If the agent is listed in Table 1, the Ontario Table, do not exceed the TWA, STEL or C set out there.
- If it is not in that table but is listed in the incorporated ACGIH Table, do not exceed the TWA, STEL or C set out there.
- If the table that applies sets a TWA but neither a STEL nor a ceiling, check the excursion rules below.
The incorporated ACGIH Table is the Adopted Values on pages 11 to 62 of the 2017 TLVs and BEIs publication. Ontario does not automatically adopt each new annual ACGIH edition. A newer recommended value does not replace an Ontario Table entry merely because it was published later.
If the agent is in neither table, section 4 sets no TWA, STEL or ceiling. The section 3 duty to take reasonably necessary measures still applies.
The ministry's combined occupational exposure limits table helps with lookups. It was updated June 26, 2025, and it brings Ontario and incorporated ACGIH values together. But the ministry says the table should not be solely relied upon, and that excursion limits are in the regulation. The same page says petroleum ether, rubber solvent and VM&P naphtha are in neither legal table, and it recommends a calculation method for them. That recommendation is not a Regulation 833 limit.
This is a practical nuisance: one table lookup may not give you every applicable exposure constraint. Keep the table entry and the relevant legal provisions together in your assessment.
Read TWA, STEL and ceiling limits correctly
The limit type matters as much as the number. Section 4 and Schedule 1 distinguish these measures:
| Limit | What it means for the assessment |
|---|---|
| TWA | Time-weighted average airborne concentration over the workday or workweek. |
| STEL | Maximum for any 15-minute period, subject to frequency and spacing rules. |
| C, or ceiling | Maximum airborne concentration at any time. |
The STEL provisions also limit exposure frequency: no more than four such exposures in an eight-hour shift. There must be at least one hour between exposures. Meeting the daily average doesn't remove these shorter-term constraints.
Schedule 1 calculates daily TWA by adding concentration-times-duration values and dividing by eight. The weekly calculation uses 40. Do not simply divide every extended shift's exposure by its actual hours. For unusual work schedules, Schedule 1 says TWA exposure may be calculated using the March 2015 IRSST guide. It does not make that method universally mandatory. See the calculation provisions.
Where the applicable table provides a TWA but neither a STEL nor ceiling, section 4 imposes excursion limits. Exposure must not exceed three times the TWA for any 30-minute period, or five times the TWA at any time. The TWA still applies. This excursion rule does not apply to agents prescribed as designated substances under O. Reg. 490/09. See section 4.
Carbon monoxide exposure limits in Ontario
Carbon monoxide shows why the table and regulation must be read together. The ministry table lists a TWA of 25 ppm, with no listed STEL or ceiling. Carbon monoxide is absent from the Ontario Table, so the incorporated ACGIH route applies.
Applying section 4's excursion rules gives:
| Exposure measure | Carbon monoxide constraint |
|---|---|
| TWA | 25 ppm |
| Any 30-minute period | 75 ppm, calculated as 3 × 25 |
| At any time | 125 ppm, calculated as 5 × 25 |
The 75 ppm value is not a listed STEL. The 125 ppm value is not a listed ceiling. Both are calculated excursion limits, which apply alongside the TWA. A low daily average doesn't cancel the peak constraints. A separate rule in section 10(5) requires a continuous carbon monoxide alarm at 5 ppm on an oil-lubricated compressor that supplies respirator breathing air. That alarm point is not the 25 ppm workplace TWA.
Assess exposure before choosing controls
Use qualified industrial-hygiene assistance to plan an assessment suited to the work. Section 6 requires recognized sampling methods. The sampling must be done by, or under the direction of, someone qualified through industrial-hygiene knowledge, training and experience.
Sampling must comply with Schedule 1. Representative sampling need not cover every minute of the workday or week. Direct-reading instruments must be used, calibrated and maintained according to manufacturer instructions. Regulation 833 does not prescribe a universal annual or six-monthly sampling interval. See section 6.
Check mixtures too. Where airborne agents have additive health effects and individual analytical results are available, Schedule 1 requires the sum of Ci/Li to be no greater than one. Each term compares an agent's concentration with its corresponding limit. Agents with substantially different health effects are assessed independently. Do not apply the additive formula to every chemical combination. See Schedule 1.
Exposure calculations take no credit for respirator use. A Skin notation also requires reasonably necessary protection against skin absorption. An airborne result below the limit does not settle those separate duties under sections 6 and 7.
Control exposure without defaulting to respirators
Section 3 identifies substitution, engineering controls, administrative controls and work practices. It also includes hygiene facilities and practices, and personal protective equipment where section 7.2 applies. Section 7.2 normally requires protection without requiring workers to use respirators. Exceptions include its subsection (2) and compliance with an applicable approved code of practice. See the control requirements.
Subsection (2) allows respirators only when substitution is unreasonable or impractical and a stated engineering-control problem also exists, including a temporary breakdown or an emergency. Where respirators are used, sections 9 to 13 address selection, written procedures, training before use, approval and protection factors. They also cover tight-fitting respirator fit testing, seal interference and manufacturer-compliant care and use. Check the respiratory provisions for the work concerned.
The July 1, 2026 amendment changed respirator provisions, including approvals and Canadian particulate-filter designations. It did not change the OEL table or the incorporated ACGIH edition. See O. Reg. 113/26.
Keep benzene, asbestos and noise in their proper regimes
For benzene, the Ontario Table lists 0.5 ppm TWA, 2.5 ppm STEL and a Skin notation. Table 1 marks that row with an asterisk, which it uses for a chemical agent also listed in Table 1 of O. Reg. 490/09. Check whether O. Reg. 490/09 applies before treating the Table 1 entry as the whole compliance program.
For asbestos, the tables list 0.1 f/cc TWA for all forms. That number is not complete asbestos compliance advice. Check O. Reg. 278/05 for relevant project, building and repair operations, and O. Reg. 490/09 for its asbestos scope and lowest-practical-level duty. Do not apply Regulation 833's generic excursion multipliers to asbestos.
Noise falls under O. Reg. 381/15. That regulation sets 85 dBA Lex,8, an eight-hour-equivalent daily exposure, not an instantaneous ceiling.
RegAffairs AI's answer on Ontario workplace chemical exposure limits links the regulatory sources behind this lookup.
For a workplace decision, check the current legal text before applying a table value. The consolidation consulted for this article displayed currency through October 28, 2026.