Heat kills new people. OSHA states that over 70 percent of heat-related deaths occur during a worker's first week, which means the most dangerous heat exposure on your site is usually a body that has not done this yet. Acclimatization is the control for that, and there are three published day-by-day schedules — one for new workers, one for experienced ones, and one written into OSHA's proposed rule.
Why the first week is the dangerous one
Acclimatization is a real physiological change, not a matter of toughness. Over days of repeated heat exposure, the body starts sweating earlier and more, at a lower salt concentration, and the heart works less hard to move blood to the skin. OSHA's guidance describes that adaptation taking roughly one to two weeks, and treats a worker as unacclimatized if they have been doing the job for less than one to two weeks.
Which is why the risk sits where it does. The worker in danger is rarely the one who has been on the roof all summer. It is the new hire on day two, the guy back from a two-week vacation, the crew that got moved from an air-conditioned plant to an outdoor yard, and the whole site during the first heat wave of the season.
Over 70 percent of heat-related deaths occur during a worker's first week — OSHA, Protecting New Workers. Not the first month. The first week.
New workers: the rule of 20 percent
The best-known schedule is the simplest. OSHA and NIOSH recommend that a worker new to working in heat do no more than 20 percent of a normal shift's heat exposure on the first day, and increase by no more than 20 percent per day after that.
| Day | Share of normal heat exposure | Roughly, on an 8-hour shift |
|---|---|---|
| Day 1 | 20% | About 1 hour 35 minutes in the heat |
| Day 2 | 40% | About 3 hours 10 minutes |
| Day 3 | 60% | About 4 hours 50 minutes |
| Day 4 | 80% | About 6 hours 25 minutes |
| Day 5 | 100% | Full shift |
Read the percentage carefully: it is a share of heat exposure, not a share of pay or of the workday. Nobody is being sent home at 9:30 a.m. The other 80 percent of day one is work in the shade, in the shop, in the trailer, on the tailgate — which is exactly where the training, the orientation and the paperwork should have been anyway.
Workers with previous heat experience: 50 / 60 / 80 / 100
Someone who has done hot work before, but not recently, does not start at 20 percent. OSHA's Heat Stress Guide, citing NIOSH, gives a shorter regimen for workers with previous experience in jobs where heat levels are high enough to produce heat stress: 50 percent on day one, 60 percent on day two, 80 percent on day three, and 100 percent on day four.
This is the schedule for the traveling ironworker who spent last month in a cooler climate, for the seasonal crew coming back in June, and for the worker returning from an injury. They have the physiology to rebuild quickly; they just have not been doing it lately.
The OSHA-NIOSH InfoSheet puts the trigger plainly: if you have someone new to the job or who has been away for more than a week, gradually increase the workload or allow more frequent breaks the first week.
What OSHA's proposed rule would require
There is no final federal heat standard. OSHA published a proposed Heat Injury and Illness Prevention rule on August 30, 2024; the informal hearing ran to July 2, 2025 and post-hearing comments closed October 30, 2025. Today heat is cited under the General Duty Clause, Section 5(a)(1) — see our guide to what actually applies right now.
Paragraph (e)(7) of the proposed regulatory text is worth knowing because it is written as a genuine choice, and its numbers differ from the NIOSH ones above.
| Who | Option A | Option B — gradual acclimatization |
|---|---|---|
| New employees, first week on the job | Apply the high-heat-trigger measures of paragraph (f) whenever the heat index is at or above the initial heat trigger during that first week | Exposure limited to no more than 20% of a normal shift on day 1, 40% on day 2, 60% on day 3, 80% on day 4 |
| Returning employees away more than 14 days, first week back | The same paragraph (f) measures during that first week back | Exposure limited to no more than 50% on day 1, 60% on day 2, 80% on day 3 |
Two details matter more than the percentages. First, Option A is not a lighter option — it trades the reduced schedule for full high-heat-trigger protections, which include a 15-minute paid break at least every two hours and active observation for symptoms. An employer that cannot cut exposure has to watch instead. Second, the proposal carries an exception: the protocols do not apply if the employer can demonstrate the employee consistently worked under the same or similar conditions within the prior 14 days. That is the clause that decides whether a transferred worker counts as new, and it puts the burden of proof on the employer.
Making a schedule that survives contact with the jobsite
The schedules are easy. Running them on a real crew is where programs fail. Five things separate the plans that work from the ones that live in a binder:
- Someone owns the list. A schedule that nobody tracks by name is not a schedule. The foreman needs to know, at the tailgate meeting, which two people on this crew are in their first week.
- The buddy is assigned, not implied. New workers get paired with an acclimatized one who knows what heat exhaustion looks like from the outside — the person in trouble is usually the last to notice.
- Hydration starts on day one. Acclimatized bodies sweat more, not less, so water demand goes up as the schedule progresses. Size it with the water intake calculator.
- Cool break areas are part of the schedule. The InfoSheet is explicit that air conditioning does not cost heat tolerance and is recommended for rest breaks. The break area is where the 80 percent of day one goes.
- Write it down. The acclimatization protocol, the trigger, the names and the dates belong in the written plan — build it with the heat illness prevention plan generator, and check the day's conditions with the heat index calculator.
One more, because it is the failure nobody plans for: a schedule that only exists for new hires misses the returning worker. Vacations, sick leave, layoffs, a week of rain, a stretch of night shifts — every one of them can put an experienced person back at the start of the ramp, in a body that no longer matches the reputation.
Bottom line
- Over 70 percent of heat deaths happen in the first week of work. Acclimatization is the control aimed straight at that number.
- New to heat: 20 percent of normal heat exposure on day one, increasing by no more than 20 percent a day.
- Experienced but not recent: 50, 60, 80, then 100 percent on day four.
- Back from more than a week away? Treat them as returning, not as experienced. The proposed rule uses 14 days and puts the burden on the employer to show otherwise.
- Air conditioning does not undo acclimatization. Cool break areas stay.