What Are the Rules for Working in Hot Conditions on Construction Sites?

Summary

Working in hot conditions is a significant occupational health risk for UK tradespeople, whether working in confined hot roof spaces in summer, in kitchens and plant rooms, or outdoors during warm weather. Despite the perception that the UK has a mild climate, heat illness and UV-related skin cancer represent real and under-managed risks.

The absence of a statutory maximum working temperature means employers have a performance-based obligation — they must take reasonable steps to manage thermal discomfort and heat risk — rather than a simple compliance threshold. This can make the obligation harder to understand but no less real. The HSE's position is clear: if employees are suffering discomfort or heat stress, action must be taken regardless of the actual temperature.

UV exposure risk is arguably more significant than acute heat illness for most UK construction workers. Outdoor workers receive significantly more UV exposure than office workers, and cumulative UV exposure is the primary cause of squamous cell carcinoma and basal cell carcinoma, both of which can be life-threatening.

Key Facts

• No statutory maximum working temperature — there is no legal maximum; the minimum for sedentary indoor work is 16°C (13°C for physically demanding work) under the Workplace (Health, Safety and Welfare) Regulations 1992
• Management of Health and Safety at Work Regulations 1999 — Regulation 3 requires risk assessment; thermal comfort is a specific risk that must be assessed
• HSE guidance — recommends a thermal comfort risk assessment when temperatures exceed 24°C indoors; WBGT (Wet Bulb Globe Temperature) measurement is the most accurate method
• WBGT (Wet Bulb Globe Temperature) — combines dry bulb temperature, wet bulb temperature (accounting for humidity), and mean radiant temperature; the most accurate single measure of thermal stress; HSE tables show alert thresholds for different work intensities
• Heat stress vs heat exhaustion vs heat stroke — a spectrum of increasing severity; heat stroke is a life-threatening emergency requiring immediate cooling and emergency services
• Hydration — CITB guidance recommends approximately 250ml of water every 20 minutes for workers in hot conditions; a total of 1 litre per hour is frequently cited for heavy outdoor work
• Acclimatisation — workers new to hot work environments need 7–14 days to acclimatise; sudden exposure to heat before acclimatisation significantly increases heat illness risk; new workers should start with shorter periods in the heat
• Skin cancer — non-melanoma skin cancer (squamous cell and basal cell carcinoma) is the second most common work-related cancer in the UK; outdoor workers have significantly elevated lifetime risk
• UV Index — daily UV Index published by the Met Office; from UV3 and above, sunscreen is recommended; from UV6+, additional protective measures required
• SPF30 minimum — HSE and CITB recommend SPF30+ sunscreen for outdoor workers; applied 20 minutes before sun exposure; reapplied every 2 hours
• Personal protective equipment for heat — shade during rest breaks is required; light-coloured, loose-fitting, UV-protective clothing is preferred over heavy protective gear in hot weather
• Rest and recovery — access to cool areas (shaded, air-conditioned welfare facilities) is required; frequency of rest breaks should increase with temperature

Quick Reference Table

Detailed Guidance

The Legal Position on Maximum Working Temperature

The UK does not have a statutory maximum working temperature. The Workplace (Health, Safety and Welfare) Regulations 1992 set a minimum temperature of 16°C for workplaces where the work is largely sedentary and 13°C for physically demanding work. There is no upper equivalent.

The TUC and trade unions have historically campaigned for a legal maximum of 30°C for most workplaces (27°C for strenuous work), but this has not been enacted in legislation.

What does exist is a performance-based obligation under the Management of Health and Safety at Work Regulations 1999 to assess and control the risk from thermal conditions. Employers must conduct risk assessments that include thermal risk, and they must take action when the risk is not adequately controlled — regardless of the specific temperature. The HSE is clear that "a reasonable temperature" must be maintained and that this is not simply measured by air temperature.

The practical implication: if workers are suffering heat discomfort or heat illness symptoms, the employer must act — even if the thermometer shows only 25°C, because humidity, radiant heat, physical workload, and PPE all contribute to thermal stress.

Measuring Thermal Stress: WBGT

Wet Bulb Globe Temperature (WBGT) is the most accurate single indicator of thermal stress in outdoor and industrial environments. It accounts for:

• Dry bulb temperature — air temperature
• Wet bulb temperature — accounts for humidity (how well sweat can evaporate)
• Globe temperature — accounts for radiant heat from sun, hot surfaces, and equipment

WBGT meters are available from safety equipment suppliers (typically £200–500) and provide a single number that can be compared against ISO 7243 thresholds for different work intensities.

For most construction work (moderate to heavy physical work), the action threshold is approximately 28°C WBGT for acclimatised workers and lower for unacclimatised workers. At or above this level, work-rest schedules should be implemented.

For practical site management without WBGT equipment, the HSE's thermal comfort checklist (available from HSE website) provides a risk assessment framework based on observable indicators.

Hydration

Dehydration significantly accelerates heat illness. Workers often arrive on site already mildly dehydrated (urine should be pale yellow; dark urine indicates dehydration). In hot conditions, sweat rates can be 1–2 litres per hour during physical work.

CITB guidance recommends approximately 250ml (a large cup) of cool water every 20 minutes for workers in hot conditions — roughly 750ml per hour during active work. Plain water is appropriate for most situations. Sports/electrolyte drinks may be beneficial for very heavy work exceeding 2 hours in high heat, but are not necessary for normal construction work durations. Alcohol, caffeine, and diuretics worsen dehydration and should be avoided before and during hot work.

Cold drinks are more effective at cooling the core body temperature than room-temperature drinks. Providing chilled water or a cooled water container on site is a meaningful risk control.

Skin Cancer and UV Protection

Skin cancer is the second most common work-related cancer in the UK (after bladder cancer in certain industries). Outdoor workers — roofers, groundworkers, scaffolders, groundskeepers, builders — receive 5–10 times more UV exposure than indoor workers over a working lifetime.

Non-melanoma skin cancer (squamous cell carcinoma and basal cell carcinoma) is directly caused by cumulative UV exposure. Melanoma (the most dangerous form) has a more complex relationship with UV but is also associated with outdoor work. All three are significantly more common in outdoor workers.

The UV Index is published daily by the Met Office. From UV3 (typically achievable in the UK from April to September, especially in southern England and at altitude), protective measures are recommended. From UV6, the risk is high and significant periods of sun exposure without protection carry real cancer risk.

Practical measures for outdoor workers:

• Apply SPF30+ broad-spectrum sunscreen (UVA and UVB protection) to all exposed skin 20–30 minutes before going outdoors
• Reapply every 2 hours; reapply after sweating heavily
• Wear a wide-brimmed hat that shades the neck (construction helmets provide minimal UV protection)
• Wear long-sleeved, lightweight UV-protective shirts where possible — UPF50+ rated clothing blocks more UV than standard cotton
• Protect the lips (SPF30+ lip balm) and eyes (UV-protective sunglasses)
• Where possible, schedule overhead work (where the face receives maximum UV exposure) for early morning or late afternoon, not midday

CITB and HSE both publish free guidance on UV protection for outdoor workers. Many employers now include sunscreen in site welfare provisions, alongside water and shade facilities.

Acclimatisation

The human body acclimatises to heat over approximately 7–14 days of regular exposure. During acclimatisation, the body increases plasma volume, begins sweating at lower body temperatures, reduces the salt content of sweat, and increases cardiac output for a given physical load. An acclimatised worker can tolerate the same conditions much more easily than an unacclimatised worker.

New workers beginning outdoor or hot indoor work should not be expected to work the same hours or intensities as acclimatised colleagues from day one. A graduated exposure programme over 7–14 days — starting at 20% of the target workload and increasing daily — significantly reduces the risk of heat illness in new workers.

This is particularly relevant at the start of a hot summer period (when an established team suddenly faces heat after months of cool weather), when workers return from absence, and when bringing on seasonal labour.

Rest and Welfare Facilities

The Construction (Design and Management) Regulations 2015 (CDM) require adequate welfare facilities on construction sites, including:

• Rest areas that provide shelter from the elements — in summer, this means shaded and/or cooled rest facilities
• Drinking water — cool, clean, and sufficient quantity
• Toilet and washing facilities

In hot conditions, access to a cool rest area is not a luxury — it is a required risk control. A cool container unit, a shaded area under trees or canopy, or access to an air-conditioned welfare cabin are all acceptable. The welfare area should be accessible without excessive travel time — if workers must walk 10 minutes to reach shade, it will not be used as frequently as needed.

Frequently Asked Questions

Can I send workers home if it gets too hot on site?

You can — and should — modify working arrangements when temperatures represent a health risk, which may include sending workers home. The decision should be based on your thermal risk assessment. There is no legal threshold that triggers mandatory site closure, but if workers are experiencing heat illness or are at genuine risk of it, the Management of Health and Safety at Work Regulations 1999 require you to act.

Is there a specific temperature at which work must stop outdoors?

No. There is no legal threshold for stopping outdoor work due to heat. The obligation is to manage the risk, which may include stopping work, relocating work to shaded areas, working different hours, or providing additional rest breaks. The threshold for action will vary with the type of work (heavy vs sedentary), the worker's acclimatisation status, and the combined effect of temperature, humidity, and radiant heat.

Does wearing high-visibility PPE make heat risk worse?

Yes. Synthetic PPE materials (high-vis vests, waterproof jackets, disposable coveralls) impair the body's ability to cool itself through sweat evaporation. They also increase radiant heat absorption. Where PPE cannot be removed, more frequent rest breaks and reduced work intensity are required to compensate. Breathable, light-coloured PPE is significantly cooler than dark, non-breathable alternatives and should be specified where possible.

Should sunscreen be provided by the employer as PPE?

The HSE's position is that employers have a duty to provide UV protection as a risk control. Sunscreen is increasingly considered a component of that duty. Many construction companies now provide sunscreen in welfare facilities or issue it with other PPE. The IOSH and CITB recommend treating sunscreen as you would any other PPE — provide it, train workers to use it, and check that it is being used.

Regulations & Standards

• Management of Health and Safety at Work Regulations 1999 (MHSAWR) — thermal comfort risk assessment

• Workplace (Health, Safety and Welfare) Regulations 1992 — minimum workplace temperature (16°C)

• Construction (Design and Management) Regulations 2015 — welfare facility requirements on construction sites

• ISO 7243 — ergonomics of the thermal environment; assessment of heat stress using WBGT index

• HSE — Thermal Comfort — HSE guidance on thermal risk assessment and management

• HSE — Working in the Sun — UV protection for outdoor workers

• CITB — Working in Heat Guidance — construction industry-specific hydration and heat guidance

• IOSH — Skin Cancer Prevention — occupational health guidance for outdoor workers

• Cancer Research UK — SunSmart — UV risk information and SPF guidance

• ppe guide — PPE selection including UV-protective clothing

• site setup — welfare facility requirements on construction sites

• lone working — elevated risk when suffering heat illness without colleagues present

• skin protection — comprehensive skin health and protection guidance