You investigate immediately, with the worker present, before anyone argues about whether the refusal is justified. Ontario's OHSA sets out the process in section 43 and it starts the moment the worker speaks up.
Short answer
The worker must “immediately” tell the supervisor or employer, and the employer or supervisor must then “investigate the situation immediately, in the presence of the worker” and a worker-side representative — a certified joint health and safety committee member if one is available, a health and safety representative, or another worker chosen by the workers. Until that investigation is complete, the refusing worker “must remain in a safe place that is as near as reasonably possible to his or her workstation.” This is informally called the first stage of a work refusal.
The grounds are specific, not a general “I feel unsafe” standard: a machine, device or thing the worker is to use or operate is likely to endanger; the physical condition of the workplace is likely to endanger; workplace violence is likely to endanger; or a provision of the Act or regulations is being contravened and that contravention is likely to endanger a worker. The worker doesn't need to prove the danger exists — a reasonable belief that one of these grounds applies is enough to start the process.
The presence requirement is not optional paperwork — the investigation happens with the worker there, alongside a worker-side representative, and the standard is “certified if possible.” That means if your project has a certified joint health and safety committee member available, they're the first call, not a fallback; see who qualifies as a certified JHSC member for what that certification actually requires.
If, after the joint investigation, the worker still has reasonable grounds to believe the danger persists, the matter escalates to a Ministry inspector rather than being resolved by the employer alone — the employer cannot simply overrule the worker's continued refusal at the first stage. Reassigning a different worker to the refused task before the process concludes carries its own notification obligations under the Act, which is why the safest sequence is always investigate first, reassign only if the Act allows it.
The most common error is treating the refusal as a disciplinary or scheduling problem to manage first and investigate later. Section 43(4) requires the report to the supervisor “immediately,” and the investigation obligation attaches the moment that report is made — not once the crew reaches a natural break, and not after a project manager has been consulted. A second error is skipping the worker's presence during the investigation because it's “faster” without them; the Act requires it specifically so the worker's account of the hazard is part of the record before anyone rules on it.
A 30-minute call is enough to tell you whether your escalation steps match section 43 or just feel like they do.
