Acclimatization means that the body gets used to working in the heat. It improves sweating, protects the heart and helps the body keep its salt. It takes days, not hours.
Why it matters most
Many heat deaths at work happen in the first days on the job or during the first heat wave of the year. The worker was not used to the heat, and worked as hard as everyone else. Lack of acclimatization is one of the top risk factors in the NIOSH guidance.
How long it takes
NIOSH says acclimatization is built up gradually over 7 to 14 days. Some benefits start in the first days of regular work in the heat. People who are less fit may need more time.
The schedule for new workers
For a new worker with no earlier experience of hot work, NIOSH recommends:
- on day 1, no more than 20 percent of the usual time in the heat,
- then add no more than 20 percent each following day,
- and supervise closely for the first 14 days.
This is sometimes called the "20 percent rule".
The schedule for experienced workers
For a worker who is experienced in hot work and has been used to the heat before, NIOSH recommends:
- 50 percent of the usual time on day 1,
- 60 percent on day 2,
- 80 percent on day 3,
- 100 percent on day 4.
Other times acclimatization matters
You can lose acclimatization. Take extra care:
- after a break from heat work, such as a vacation, illness or time off,
- when the weather turns suddenly hotter, in a heat wave,
- when you move to a hotter job, indoor hot area or climate,
- when you start using heavier protective clothing.
Time in air conditioning at home or at work does not undo acclimatization for a worker who is actually working in the heat each day.
Rules about the first days
Some states put this into rules. For example, California's standards require employers to closely observe workers during a heat wave and new employees in the first 14 days in high-heat work.
What you and your employer can do
- Tell your supervisor if it is your first day or first week in hot work, or you are coming back after time away.
- Ask for lighter tasks, more breaks and more water during the first days.
- Employers should plan the schedule before the new worker arrives, and never expect a new worker to match a veteran's pace on day one.
- Employers should treat the first hot days of the season as a risk for everyone.
A word on fitness, age and health
Being fit helps, but it is not a substitute for acclimatization. A strong, young worker who is new to the heat can still become ill. Older workers, people with some health conditions and people on some medicines may need more time, and some may need to talk to their doctor about heat work. Employers should not rank workers by who "can take it".
Acclimatization is not protection from everything
An acclimatized worker still needs water, rest and shade. Acclimatization helps the body cope, but it does not make a worker immune. Extreme conditions can still cause heat stroke.
What to remember
- Build up gradually over 7 to 14 days.
- New workers: 20 percent on day 1, then no more than 20 percent more per day.
- Experienced workers: 50, 60, 80, 100 percent over four days.
- Be careful after time off and during heat waves.