Acclimatisation
Quick answer
Acclimatisation is the gradual process — usually 7 to 14 days — that the body needs to safely handle work in hot conditions. New employees, returning employees and anyone assigned to a hotter role are at the highest heat-illness risk in that window, and a written acclimatisation plan is one of the single highest-impact safety controls an employer can put in place.
What acclimatisation is
Given consistent exposure to heat, the body adapts. Sweat production increases and starts earlier. Cardiovascular strain drops. Perceived effort at a given workload falls. The person can safely handle heat levels that would have made them ill in their first week.
This adaptation is not instant. It typically requires 7 to 14 days of gradually increasing exposure, and it can be lost after even a week away from heat exposure — which is why post-holiday and post-illness periods deserve the same care as brand-new hires.
Why it matters at work
Data from OSHA and multiple national safety bodies consistently shows that a disproportionate share of serious workplace heat illness happens in the first days on a hot job. Many of those cases are entirely preventable with a written acclimatisation schedule.
A simple 'rule of 20%' schedule
A widely used pattern: on day one, the new worker performs no more than 20% of the normal duration of heavy heat-exposed work. Each day, the duration increases by up to 20%, reaching a full day by roughly day five. Returning workers who have been away for more than a week follow a compressed version of the same schedule.
Common mistakes
Treating experienced hires as already acclimatised
A skilled worker returning from two weeks of holiday is not acclimatised on day one. Their skill is intact; their heat tolerance is not.
Frequently asked questions
- How long does acclimatisation take?
- Typically 7–14 days of gradually increasing exposure. Younger, fitter workers may adapt faster; older workers or those with health conditions may take longer.