What is heat stress?
The human body maintains its internal temperature around 37 degrees Celsius through regulatory mechanisms, primarily sweating and increased blood flow to the skin. When the heat generated by activity and received from the environment exceeds what these mechanisms can dissipate, the internal temperature rises. Dehydration, lack of acclimatization, waterproof or protective clothing, certain illnesses and medications, and age increase vulnerability.
The effects are graduated: heat rash and cramps, heat syncope, heat exhaustion (weakness, nausea, headache, profuse sweating), and heat stroke, characterized by very high body temperature, altered consciousness, and failure of thermoregulation, which requires immediate medical attention. Heat also increases the risk of accidents due to loss of attention and concentration.
Thermal stress should not be confused with thermal discomfort, which is regulated by Annex III of Royal Decree 486/1997 for enclosed spaces (temperatures between 17 and 27 degrees Celsius in sedentary work). Thermal stress is a health risk that requires specific assessment, especially in outdoor work, in industries with heat sources (foundries, furnaces, glass, kitchens, laundries), and in tasks involving physical exertion or the use of protective clothing.
Risk assessment
- WBGT Index. Globe and wet-bulb temperature, calculated from the natural wet-bulb temperature, globe temperature and air temperature; it is compared with reference values that depend on the metabolic consumption of the task and the degree of acclimatization, according to the UNE-EN ISO 7243 standard. The NTP 922 and 923 and the NTP 1189 of the INSST describe the method.
- Metabolic consumption. Estimation of the physical workload according to the UNE-EN ISO 8996 standard; the higher the consumption, the lower the WBGT reference value.
- Clothing. Index corrections according to the insulation and permeability of clothing and protective equipment.
- More detailed methods. When the WBGT indicates risk, the estimated thermal overload (PHS index, UNE-EN ISO 7933 standard) allows the calculation of maximum exposure times and water losses.
- Individual factors. Acclimatization, hydration, health status, medication, age and pregnancy, considered in health surveillance.
- Outdoor conditions. Weather warnings from the State Meteorological Agency, which since 2023 have legal effects on the organization of outdoor work.
Regulatory framework and measures for 2023
- Royal Decree 486/1997, Annex III. Environmental conditions of enclosed workplaces and the obligation to protect outdoor work from inclement weather.
- Royal Decree-Law 4/2023, of May 11. Adds to Article 4 of Royal Decree 486/1997 the obligation to adopt appropriate measures in the face of adverse weather phenomena, including extreme temperatures, derived from the risk assessment, and prohibits certain work during the hours in which there is a warning of risk or extreme risk issued by the State Meteorological Agency or the competent regional body, when protection cannot be guaranteed.
- Law 31/1995, articles 15, 16 and 25. Principles of preventive action, risk assessment and protection of particularly sensitive workers .
- Royal Decree 1627/1997 and Royal Decree 1561/1995. Construction works and special working days, with rules on the organization of work in adverse weather conditions.
Organizational application: how to manage heat
- Identify positions with exposure to heat (outdoors, radiant sources, physical exertion, protective clothing) and assess the risk using the WBGT index or equivalent methods.
- Establish a seasonal protocol for heat episodes with action thresholds linked to weather warnings and internal measurements.
- Adapt the organization: schedules that avoid peak hours, rotation of tasks, breaks in cool or shaded areas, work pace and reduction of effort.
- Acting on the environment: shade, ventilation, shielding of radiant sources, cooling of cabins and premises.
- Ensure access to fresh water, lightweight and breathable clothing where compatible with protection, and gradual acclimatization for new or returning personnel.
- Provide training on symptoms, first aid for heat stroke and mutual monitoring, and integrate particularly sensitive individuals into health surveillance.
- Record measurements, decisions made in response to warnings and episodes of thermal overload, and review the protocol each season.
Preventive management software allows you to record measurements, activate protocols in response to alerts, document the measures taken by center and shift, and maintain traceability of training and incidents.
Limits and common mistakes
- Evaluate heat using only air temperature, without considering humidity, radiation, activity, and clothing.
- Confusing the comfort values of Annex III of Royal Decree 486/1997 with thermal stress criteria.
- Continuing outdoor work during extreme risk warnings without protective measures.
- Relying on individual hydration for prevention without acting on the organization and the environment.
- Forget about acclimatizing new hires, temporary staff, and those returning from vacation.
- Failure to provide training in recognizing heat stroke and in immediate action.
The reference values for the WBGT index and the measurement procedures should be consulted in the INSST Prevention Technical Notes and in the current technical standards.
Practical example
Situation: A gardening and green area maintenance company with 90 people is preparing for the summer season.
- Evaluation. WBGT measurements in brush clearing tasks with motor and protective clothing, which show risk in the middle hours of July and August.
- Protocol. Intensive morning shift, reassignment of more strenuous tasks first thing in the morning, hourly breaks in the shade, fresh water in all vehicles, and suspension of brush clearing with a red or orange alert when protection cannot be guaranteed.
- People. One-week acclimatization for new hires, attention to workers with health conditions reported to the prevention service, and training in symptoms and first aid.
- Monitoring. Daily record of notices and decisions, heat incident indicators and review of the protocol at the end of the season.
Regulatory framework in Spain
- Royal Decree 486/1997, article 4.4 and annex III . Environmental conditions and measures in the face of adverse weather phenomena, including extreme temperatures.
- Royal Decree-Law 4/2023, of May 11. Prevention of occupational risks in episodes of high temperatures; amendment of Royal Decree 486/1997.
- NTP 922 of the INSST . Thermal stress and thermal overload: risk assessment (I).
- NTP 1189 of the INSST . Assessment of the risk of heat stress: WBGT index.
- Law 31/1995, articles 15, 16 and 25. Preventive principles, risk assessment and especially sensitive workers.
The technical standards UNE-EN ISO 7243 (WBGT index), UNE-EN ISO 7933 (estimated heat load), and UNE-EN ISO 8996 (metabolic consumption) are the methodological references. In Colombia, the assessment of heat stress is integrated into the Occupational Health and Safety Management System (SG-SST) and uses the same international indices.
