Heat acclimatisation for outdoor workers: a practical 7-day plan
New and returning workers are most at risk in their first days in the heat. A simple plan to build them up over at least 7 days.
A worker's first few days in hot outdoor conditions are the most dangerous. The body has not yet learned to sweat earlier, sweat more and hold on to salt. That adaptation is called heat acclimatisation, and it takes about a week.
MOM expects employers to run an acclimatisation programme of at least 7 days for workers who need it. Here is who needs it and how to run one without disrupting the work schedule.
Who needs to be acclimatised?
According to MOM, acclimatisation applies to workers who are:
- newly assigned to outdoor work
- new to Singapore, or back from a cold-climate country
- back from leave of more than a week
- recovering from a prolonged illness
The last two are easy to miss. A seasoned worker coming back from two weeks of home leave has lost much of their heat tolerance and should be treated as unacclimatised.
How acclimatisation works
MOM's approach is to increase the time spent in the heat gradually, with frequent rest and water breaks along the way. Most people adapt within 7 to 10 days.
MOM does not set a fixed daily schedule. One widely used approach, from the US National Institute for Occupational Safety and Health (NIOSH), is to start new workers at about 20% of normal heat exposure on day one, then add no more than 20% each day.
A simple 7-day plan
Adapt this to your shifts and the type of work.
| Day | Time on hot outdoor work | Notes |
|---|---|---|
| 1 | About 20% of a normal shift | Lighter tasks, rest often, watch closely |
| 2 | About 40% | Keep pairing with an experienced buddy |
| 3 | About 60% | Introduce heavier tasks in cooler hours |
| 4 | About 80% | Normal tasks, extra rest breaks |
| 5 – 7 | Full shift | Normal breaks; keep the buddy check |
Workers returning from leave or illness can usually move faster, for example starting at half a shift, but still need the full week of extra attention.
For the rest of the shift, give them shaded or indoor tasks so they stay productive.
Make it easy to run
- Flag who needs it when rostering: new hires, returning workers and anyone back from sick leave.
- Keep a simple log: name, start date, hours outdoors each day.
- Pair them with a buddy who knows the signs of heat illness.
- Tell supervisors which workers are still acclimatising, so they are not given the hottest jobs first.
- Pause and restart if the worker falls ill during the week.
Watch for warning signs
Unacclimatised workers are more likely to show early signs of heat illness, such as dizziness, headache, nausea, cramps or unusual tiredness. Stop work, move them to shade and cool them down. Our guide to recognising heat exhaustion and heat stroke explains what to do next.
How HeatAware helps
The HeatAware app keeps the training and acclimatisation records together with WBGT readings and alerts, so you can show which workers were built up and when. Size your kit and take the 17-point check: acclimatisation is point 2.
Frequently asked questions
How long does acclimatisation last?
It fades without heat exposure. After more than a week away, MOM treats the worker as needing acclimatisation again.
Do experienced workers need it?
Not if they work in the heat every day. They do if they return from more than a week of leave, from a cold climate, or from a long illness.
Does acclimatisation replace rest and water breaks?
No. Acclimatised workers still need all the measures for the WBGT band.
General guidance based on MOM's FAQs on Heat Stress Measures for Outdoor Work. The sample schedule is illustrative; adjust it to your risk assessment.