A heat acclimatization schedule for new and returning workers

What is heat acclimatization?

Heat acclimatization is the body’s gradual adaptation to hot work. OSHA defines acclimatization as the body gradually adapting to tolerate higher levels of heat stress. NIOSH describes beneficial changes, such as increased sweating efficiency, after gradual exposure to a hot environment.

OSHA’s page on new workers lists the changes. Acclimatized workers sweat at a higher rate, lose less salt in sweat, keep a lower body temperature and heart rate, and move more blood to the skin. Unacclimatized workers do none of this well. See the acclimatization glossary entry for a short definition.

Why does acclimatization matter so much for new workers?

Because new workers are the ones who die. OSHA says “Almost half of heat-related deaths occur on a worker’s first day on the job or a worker’s first day back after an extended absence.” OSHA adds that over 70 percent of heat-related deaths occur during a worker’s first week.

Fitness does not protect a new hire. OSHA says “Even young, seemingly “fit” workers can be susceptible to heat-related illness.” The risk also has a people side: new workers may push too hard, skip breaks or hide symptoms to avoid looking lazy.

What is the 20 percent rule for new workers?

Start at 20 percent of the normal time in the heat, then add 20 percent a day. OSHA and NIOSH recommend this for new workers. NIOSH says “No more than 20% of the usual duration of work in the heat on day 1,” with “No more than 20% increase on each additional day.”

OSHA gives a worked example for an 8-hour day: new workers should work “no more than 1 hour and 40 minutes (20 percent of 8 hours) on their first day in the heat.” The rest of the day should be without heat stress. They should get at least one rest break during the heat period.

The 20 percent and the 1 hour 40 minutes figures come from NIOSH and OSHA. We did the math for days two through five.

Many construction crews work 10-hour days. Apply the same percentages to the longer shift. This is our arithmetic on the NIOSH percentages:

OSHA says new workers can spend the rest of the first-day shift without heat stress, so plan where the rest of each shift goes. OSHA says that by following the rule, new workers “will be working a full schedule by the end of their first week.”

What schedule works for workers with heat experience?

Experienced workers can ramp faster. NIOSH gives a four-day plan: 50 percent of the usual duration on day 1, 60 percent on day 2, 80 percent on day 3 and 100 percent on day 4. Use this plan only for people who have worked recently in the same kind of heat.

NIOSH says the whole process takes 7 to 14 days. It adds that “Workers who are not physically fit need more time to fully acclimatize.” OSHA says the 20 percent rule should protect most workers who are physically fit and have no medical problems, and some need up to 14 days. When in doubt, give workers more days.

Who else needs a ramp-up besides new hires?

Anyone whose heat tolerance has dropped. OSHA lists four groups: new, temporary or existing workers who start new work in warm or hot places, workers returning after an absence of one week or more, workers working through spring and early summer as heat first rises, and workers on days much warmer than the days before.

OSHA says workers can lose heat tolerance during vacation, sick leave and winter. For them, use the same steps as for new workers and keep the added protections for at least one week. OSHA’s planning page adds that temporary workers may need closer supervision, and that people back from leave of more than two weeks may be at higher risk.

Our acclimatizing new hires in Texas post puts this into a summer start plan.

What should the work look like during the ramp-up?

Keep the work real but short. OSHA says to “reduce the duration of the work but not the intensity of the work.” A new brick layer should lay bricks during the first week, because light work may not prepare the body for the job.

During the ramp-up:

1. Give longer, more frequent rest breaks. NIOSH says new and unacclimatized workers need longer, more frequent rest periods.
2. Use a buddy system. Do not let new workers work alone.
3. Teach the signs of heat illness and the importance of rest and water. Use a work/rest schedule for the heat hours.
4. Let any worker with symptoms stop work, start first aid and never leave them alone.
5. Watch for the lag effect. OSHA warns that unacclimatized workers who feel fine on day one can get sick on a later day.

What mistakes slow down a safe ramp-up?

Most mistakes come from pressure to look tough. OSHA says “The first days of a new job or a new work activity can contain unique psychological and behavioral pressures.” New workers may push too hard to prove themselves, skip breaks so no one thinks they are lazy, or not know who to tell when they feel bad.

Watch for these on your site:

• A new hire who refuses a break. Make the break part of the schedule, so no one has to ask.
• A crew that races to keep up with a veteran. Pair new hires with a calm buddy.
• A supervisor who skips the ramp-up in a rush. Put the schedule in the daily plan.
• A returning worker who is treated like a veteran after a long absence.
• No one on site knows the 911 steps.

Keep a simple log: worker name, day number, hours in the heat, breaks taken and symptoms reported. It takes a minute and shows the plan ran.

Does air conditioning or a few days off undo acclimatization?

Not quickly. NIOSH says “Acclimatization can be maintained for a few days of non-heat exposure.” It also says “Taking breaks in air conditioning will not affect acclimatization.” So cool break areas are safe for the plan.

Longer breaks matter. After an absence of a week or more, treat the worker as unacclimatized and restart a ramp-up. OSHA says to assume workers are unacclimatized if they have been doing the job for less than one to two weeks.

How do hydration and electrolytes fit into acclimatization?

Water stays on every day of the ramp-up. Acclimatized workers lose less salt in sweat, but they still sweat, and new workers have not adapted yet. OSHA says water cannot replace electrolytes and recommends electrolyte drinks on jobs longer than two hours.

JMAR Solutions’ Rapid Restoration Drops are a dietary supplement that replaces electrolytes lost in sweat and supports hydration. They do not replace a ramp-up schedule, rest or shade. JMAR’s hydration programs include onsite toolbox talks and coaching for new-hire heat starts. For a full program, see our heat stress program guide and the heat stress prevention pillar.

If a worker shows confusion, slurred speech or collapse, call 911. That is a heat stroke emergency.

Next step

Want to roll out a new-hire heat plan this season? Book a 15-minute demo with JMAR Solutions, or ask for a free sample for your first toolbox talk.