During an early spring heat wave in southern California, a construction crew mobilized onto a jobsite, where temperatures exceeded 100°F by early afternoon. Water and shade were available, but within hours, a worker collapsed from a heat-related illness. The issue didn’t stem from lack of access to resources like water and shade, but rather a lack of preparation.

This scenario is not hypothetical. A recent early-season heat wave in Southern California pushed temperatures into the triple digits weeks ahead of typical summer conditions. Many crews were not yet acclimatized, and workers were suddenly exposed to full workloads in extreme heat. It’s a reminder that heat illness risk doesn’t wait for the calendar; it shows up whenever the conditions arrive.

In construction, manufacturing, and other high-exertion industries, heat exposure is not an abstract risk; it is predictable, measurable, and something we see season after season. Yet many heat illness prevention efforts still focus primarily on what happens during the work shift: water, rest, and shade. These controls are essential, but by the time you rely on them, you’re already reacting. The most effective heat illness prevention programs begin days in advance, preparing the worker, not just the worksite, for heat exposure. From an industrial hygiene standpoint, this means focusing on acclimatization, hydration status and workforce readiness before temperatures peak or workloads intensify.
 

A predictable hazard that is often underestimated

Heat-related illnesses occur when the body cannot adequately dissipate internal heat generated by metabolic work and environmental conditions. In construction and industrial settings, contributing factors often include:

• Heavy physical exertion performed during labor-intensive jobs (e.g., concrete work, roofing, steel erection).
• Impermeable or semi-impermeable personal protective equipment.
• Radiant heat from surfaces such as asphalt, metal or furnaces.
• High humidity, an environment that limits evaporative cooling. 

The Occupational Safety and Health Administration (OSHA) recognizes heat as a serious workplace hazard. It continues to emphasize employer responsibility under the General Duty Clause to provide a workplace free from recognized hazards (OSHA, n.d.); however, enforcement and compliance alone do not prevent heat illness. Preparation does, and that’s where many programs fall short.
 

Acclimatization: The Most Overlooked Control

One of the most significant risk factors for heat illness is lack of acclimatization. Workers who are new, returning from time off, or reassigned to hotter environments are at the highest risk. In fact, the majority of heat-related fatalities occur within the first few days of exposure (CDC, 2020). Acclimatization is a physiological adaptation process that improves the body’s efficiency in managing heat. Benefits include:

• Earlier onset of sweating;
• Increased sweat rate;
• Reduced heart rate and core temperature during work; and 
• Improved electrolyte balance. 

The National Institute for Occupational Safety and Health (NIOSH) recommends a gradual increase in exposure over seven to 14 days (NIOSH, 2016). For construction crews mobilizing onto a new site or workers returning after a week away, this process is critical. 

In practice, acclimatization only works when it is structured and enforced; otherwise, production pressures tend to override it. Heat illness prevention cannot be left to informal practices or worker self-regulation. Production pressures often push crews to full workload immediately, but this is precisely when the risk of a heat injury is highest.
 

Hydration: It Starts Before the Shift

Hydration is often addressed with reminders to “drink water frequently,” but this guidance is incomplete if it only applies during the workday. Workers do not begin their shift in a neutral state; they arrive either hydrated or already in deficit. Dehydration can begin the day before due to inadequate fluid intake, alcohol consumption, or prolonged heat exposure without recovery. Even mild dehydration can increase cardiovascular strain, reduce sweat production efficiency, and impair cognitive function and decision-making. On construction and industrial jobsites, this can easily translate directly to increased injury risk (Alahmad et al., 2025). 

An effective hydration strategy includes pre-shift hydration, consistent intake, and electrolyte replacement. This means encouraging workers to begin hydrating before they arrive onsite, supporting small, frequent fluid intake throughout the shift, and considering electrolyte replacement during prolonged or high-intensity work. In practice, simply providing water is not enough. Accessibility, temperature and work pacing all influence whether workers actually drink it. 

Prevention is also a function of how work is planned. In these environments, this may include scheduling heavy tasks (e.g., rebar tying, roofing, confined space work) during cooler parts of the day, rotating crews to reduce continuous exposure, and adjusting productivity expectations during heat events. These are administrative controls, but they are most effective when paired with a workforce that has been properly acclimatized and hydrated.

Personal protective equipment can significantly increase heat stress by limiting heat dissipation. In industrial settings, this may include flame-resistant clothing, chemical protective suits, and respiratory protection. In the field, this means evaluating the combined heat burden, not just the temperature, but also the workload and what the worker is wearing. The American Conference of Governmental Industrial Hygienists (ACGIH) provides guidance on Threshold Limit Values (TLVs) for heat stress that account for these variables (ACGIH, 2023). Where feasible, engineering controls such as ventilation, shielding, or process modification should be implemented to reduce heat load at the source.
 

Worker Factors: Conditioning, Sleep and Health

Individual susceptibility plays a significant role in heat tolerance. Workers who are deconditioned, sleep-deprived, or managing underlying health conditions may be at increased risk. Effective heat illness prevention programs should educate workers beyond the jobsite and into their daily habits. Workers who maintain a reasonable level of physical condition are better equipped to tolerate heat stress, as their cardiovascular systems are more efficient at managing both workload and temperature. Equally important is adequate sleep and recovery between shifts. Fatigue impairs the body’s ability to regulate heat and can reduce a worker’s awareness of early warning signs, increasing the likelihood of serious outcomes. Programs should also address the potential impact of medications on heat tolerance. Certain prescriptions and over-the-counter drugs can affect hydration, heart rate or thermoregulation (CDC, 2025).

Educating workers on these factors empowers them to recognize personal risk and make informed decisions before entering a hot work environment. Education is particularly important in industries with seasonal workforces, where workers may not be physically prepared at the start of the season.
 

OSHA Alignment: From Compliance to Prevention

OSHA’s current emphasis on heat illness prevention includes key elements such as water, rest, shade and training. While these are critical, they represent minimum expectations. A comprehensive program aligned with OSHA principles should also include documented acclimatization procedures, pre-shift and pre-season planning, supervisor accountability for implementation, ongoing evaluation and adjustment, and worker education. Heat illness prevention should be treated with the same rigor as other occupational hazards: planned, documented and continuously improved.

Day-of controls remain essential. Workers must have access to water, shaded rest areas, and be monitored for early signs of heat illness. However, these controls are significantly more effective when layered on top of a workforce that is already prepared. Heat illness prevention is not a single intervention; it’s a system. When acclimatization, hydration, planning and education are in place, the system works. When they are absent, even well-intentioned controls may fall short.

Workers will be exposed to heat. The real question is whether they are prepared for it before their shift begins. 

References

1. Alahmad, B., Kessler, W., Alwadi, Y., Schwartz, J., Wagner, G. R., & Michaels, D. (2025). A nationwide analysis of heat and workplace injuries in the United States. Environmental Health, 24(1), 65. https://doi.org/10.1186/s12940-025-01231-1

2. American Conference of Governmental Industrial Hygienists [ACGIH]. (2023). 2023 TLVs and BEIs: Threshold Limit Values for Chemical Substances and Physical Agents & Biological Exposure Indices. ACGIH.

3. Centers for Disease Control and Prevention [CDC]. (2020). Heat stress: Occupational safety and health. https://www.cdc.gov/niosh/topics/heatstress/

4. Centers for Disease Control and Prevention [CDC]. (2025). Heat and medications: Guidance for clinicians. https://www.cdc.gov/heat-health/hcp/clinical-guidance/heat-and-medications-guidance-for-clinicians.html

5. National Institute for Occupational Safety and Health [NIOSH]. (2016). Criteria for a recommended standard: Occupational exposure to heat and hot environments. U.S. Department of Health and Human Services.

6. Occupational Safety and Health Administration [OSHA]. (n.d.). Heat illness prevention campaign. https://www.osha.gov/heat