Five of the ten most dangerous summer dates for outdoor workers fall in June, not July or August as most assume. The finding challenges the widespread belief that peak summer months pose the greatest heat risk to Britain’s tradespeople.
St Albans recorded 47 days where temperatures hit dangerous levels during working hours, making the Hertfordshire city the riskiest location in Britain for outdoor workers. City of London followed with 45 days, whilst Oxford and Cambridge each logged 37.
The analysis, conducted by insurance provider Protectivity, examined historical weather data across UK cities between 2023 and 2025. Researchers flagged any day where temperatures reached 27°C or above for at least two hours between 6am and 6pm—the threshold at which the TUC recommends strenuous outdoor work should cease.
What emerged surprised even the researchers. August 12th ranked as the single most dangerous working day, recording 61 instances of hazardous temperatures across the three-year study period. But June claimed five slots in the top ten most dangerous dates: the 13th, 20th, 21st, 25th and 30th.
“Five of the ten most dangerous dates in our dataset fall in June, and yet June is often when sites are running at full capacity with no particular heat protocols in place,” said Chris Trotman, underwriting manager at Protectivity.
The timing matters. By mid-June, construction sites typically operate at full capacity, with few heat safeguards in place. Most companies reserve heat protocols for late July and August, leaving workers exposed during a statistically more dangerous period.
Britain lacks any legal maximum working temperature. While other European nations enforce upper limits beyond which outdoor work must pause, UK employers need only maintain a ‘reasonable’ temperature—a term with no legal definition and no enforceable ceiling. An estimated 100 million working hours are lost to heat stress across the British labour force each year, according to research by the Physiological Society.
Urban areas face compounded risk. The heat island effect—where concrete, tarmac and traffic push temperatures several degrees higher than surrounding areas—explains why cities dominate the rankings. Birmingham recorded 33 dangerous days, Gloucester 32, Worcester 32, Winchester 31 and Peterborough 29. Chelmsford, with 34 days, completed the top five behind St Albans, London, Oxford and Cambridge.
Last summer became the hottest on record, with temperatures peaking at 35.8°C. Forecasters predict similarly intense conditions this year.
For James Crame, health and safety advisor at Start Safety, the data underscores failures in current workplace practices. “Telling workers to drink water is not a heat policy,” he said. “A compliant heat management approach requires documented rest rotation schedules, with frequency increasing as temperature rises. Workers should be encouraged to hydrate every 15-20 minutes by drinking small amounts frequently, rather than large volumes infrequently. Shade structures or site welfare units should be positioned to allow genuine rest, away from radiated heat from plant, machinery and structure.”
Crame outlined five critical steps employers should take before temperatures climb further.
First, conduct a documented heat risk assessment. “Every site should have a documented heat risk assessment that identifies which tasks, roles and workers are most exposed. This should be updated at the start of each summer season and whenever new workers join. Key factors to assess include direct sun exposure by task type, PPE heat burden per role, access to shade and water, worker age and health status, and acclimatisation status.”
Second, implement acclimatisation schedules for new and returning workers. “The majority of heat fatalities occur in the first few days of hot conditions. New workers, those returning from holiday or sick leave, and workers new to physically demanding roles must be given a structured acclimatisation period by gradually increasing their exposure and workload over five to seven days.”
Third, reschedule strenuous tasks away from peak heat hours. “Outdoor task scheduling should be adjusted during periods of forecast heat. Strenuous physical work should be moved to early morning (before 10am) or late afternoon (after 4pm) wherever possible. UV radiation and ambient temperature are highest between 11am and 3pm so these hours should be avoided.”
Fourth, establish structured work-rest-hydrate regimes rather than leaving decisions to individual workers.
Fifth, review PPE for summer conditions. “PPE selection in summer requires a review that asks if lighter, breathable alternatives could be used without compromising the required protection level. Vented hard hats, moisture wicking base layers, lightweight hi-vis, breathable gloves, and UV protective eyewear can all reduce heat burden without sacrificing compliance. Workers should be explicitly encouraged to remove PPE during rest breaks where it is safe to do so, to allow the body’s cooling systems to function.”
The construction industry already records the highest number of worker fatalities of any sector, according to Health and Safety Executive data. Heat stress adds another layer of risk to an already dangerous profession.
Self-employed tradespeople face particular vulnerability. They represent the largest self-employed workforce of any sector in the UK, yet have no employer to mandate rest breaks or enforce heat protocols.
“Self-employed tradespeople in particular, who make up the largest self-employed workforce of any sector in the UK, have no employer to mandate rest breaks or enforce a heat policy on their behalf,” Trotman noted. “The data makes clear this isn’t a niche risk and for tradespeople working outdoors right now, this is a very present and very real danger.”
Trotman highlighted the geographic spread of the risk. “What makes this research particularly striking is that the risk isn’t concentrated in one place or one month and it’s actually spread across cities many wouldn’t necessarily flag as heat hotspots, and it also starts earlier in the summer than most people would assume.”
The research analysed cities with populations exceeding 100,000, using weather data from timeanddate.com. Only days where temperatures sustained 27°C or higher for at least two hours during standard working hours qualified as dangerous.
June 20th emerged as the second most dangerous individual date, with 43 recorded instances of hazardous conditions. June 13th followed with 42, whilst July 11th recorded 41. June 30th logged 37 dangerous instances, August 11th had 36, and June 21st registered 35.
For workers across St Albans, London, Oxford and dozens of other cities, the message is clear: the danger starts earlier than the calendar suggests. By the time August arrives, June will already have posed the greater threat.