Ozone produced by the action of sunlight on vehicle pollution causes smog, and is a particular threat to asthma sufferers, contributing to tens of thousands of serious attacks each year and hundreds of deaths.
Asthma sufferers should avoid outdoor exercise in the afternoon, and cut unnecessary car journeys.
Research has shown that an increase in pollution levels during sunny weather is followed by a rise in emergency hospital admissions. The maximum risk occurs when hot weather and high pollution levels coincide with thunderstorms during the pollen season.
The first hot spell of the year is always enticing, but smog is not the only hazard. The temptation is to strip off and soak up the sun but even though it still lies low in the sky it can deliver a vicious burn to pale skin that has remained covered up for the past six months.
Melanin is the pigment released by the action of the sun on the skin that helps it tan and provides protection from burning.
The advice to apply sunscreen and cover up in the sun is never more important than during the first few days of hot weather. Avoiding burning is especially important for children under 15. Research shows that severe sunburn before this age increases the risk of melanoma, the most serious form of skin cancer. People with lots of moles, red hair, fair skin and a family history of skin cancer should take extra care.