Today's forecast: hot, with rising levels of smog, asthma and hay fever

Soaring temperatures have prompted a rash of medical alerts, amid fears that smog will choke British cities this summer.

As the country recorded its hottest 12 June for more than 80 years, people were warned by both the Met Office and the Department for Environment, Food and Rural Affairs (Defra) - the government department responsible for monitoring air quality - to take "sensible precautions" against smog, such as avoiding outdoor exercise in the afternoon, and cutting unnecessary car journeys. For Britain's thousands of asthma sufferers, the situation is likely to get worse. Climate change predictions suggest that, with global warming, Britain is in line to experience summer episodes of such extreme heat that they may lead directly to deaths.

Global warming has already been blamed for the rising numbers of Britons suffering from hay fever. Last month, a UK pollen specialist said the pollen from trees and grasses that produces allergic reactions in millions of people is steadily increasing with rising temperatures.

Professor Jean Emberlin, director of the National Pollen and Aerobiology Research Unit, said pollen seasons are lengthening, and the pollen itself is provoking a more powerful reaction among hay fever sufferers.

The Met Office yesterday said high temperatures will continue for the rest of June, increasing the likelihood of more smog and air pollution incidents that may be widely damaging to health.

Professor Martyn Partridge, chief medical adviser to Asthma UK, said atmospheric conditions over the next few days looked set to repeat those of 24 June 1994, when Britain suffered its worst epidemic of asthma attacks, as thunderstorms struck during a heatwave. In London that year, 640 patients were taken to accident and emergency departments in a period of 30 hours, 10 times the usual number, as the thunderstorm crossed the capital.

"We face a similar scenario now. We are at the end of a period of hot weather and there is lots of pollen in the atmosphere," Professor Partridge said.

The first episode of global warming's impact has already been experienced in France, where in the first fortnight of August, 2003, soaring temperatures brought about nearly 15,000 unexpected deaths.

The French public, at first, treated the situation as a Government failure to cope but when scientists analysed the temperature data, they realised it was the hottest summer on record in Europe. Researchers have attributed it directly to climate change - the first such case.

Britain caught the edge of that heatwave and registered a record of its own, with the official UK air temperature passing 100F for the first time on August 10 2003. That day, the UK temperature record jumped from 98.8F to 101.3F -- an astonishing two-and-a-half degree leap.

Yesterday, the temperature in Central London nearly reached the nineties, hitting 89.8F, or 32.1C. It was as record of its own - the hottest 12 June in the UK since 1925. And there will be more. Temperatures in much of Britain are likely to be "above normal and hot" for the remainder of June, according to the Met Office's UK Further Outlook Forecast issued yesterday.

Thunderstorms are likely in some parts of Britain this week, and that may mean an increased risk for Britain's 5 million asthma sufferers.

Younger people and those with mild asthma are at particular risk. The young are more likely to be outside exercising and drawing polluted or pollen-laden air into their lungs. People with mild asthma are less likely to use their inhalers or other medication consistently and may thus be at greater risk when atmospheric conditions suddenly change.

Professor Partridge said research during the past 20 years had shown that an increase in pollution levels during sunny weather was followed by a rise in emergency hospital admissions. The maximum risk occurred when hot weather and high pollution levels coincided with thunderstorms during the pollen season - as forecast for this week.

"There is loads of pollen that has floated up high in the heat and when you get a radical temperature inversion, as happens in thunderstorms, it comes rushing back down," he said. "The starchy pollen granules are broken up by the moisture so they can be drawn deep into the lungs. Normally, the pollen grains are large enough to be filtered out and trapped in the nose."

The best way to deal with the threat was to avoid it, he said. "If you find exercising on the outskirts of town in the evening brings on your asthma then do it elsewhere or at another time."

If the threat was unavoidable, such as in a thunderstorm, then medication was the only defence in the event of an attack.

Professor Partridge said every sufferer should have an asthma action plan spelling out the warning signs of an attack and telling them what to do. But though it was recommended in guidelines issued 16 years ago, only one in five sufferers had one.

"There are 5.2 million asthma sufferers in the UK and there is no doubt that thunder and increasing pollution make asthma worse," he said.

Smog - smoky fog - used to be the scourge of British cities, until the Clean Air Act transformed the situation half a century ago.

The term was applied to winter "pea-soupers" - the combination of smoke from coal fires and fog in cold temperatures - and 4,000 died in December 1952 when smog descended on London.

Smog today is different - the combination in summer of bright sunlight and nitrous oxide from car exhausts to produce ozone, which is particularly dangerous to those suffering from asthma. Add in increased pollen counts and the situation is much worse.

One sufferer, Sue Edwards, 33, from Audley, Stoke-on-Trent, said: "I suffer from severe asthma, it's so bad that I have to work from home because an office environment would trigger attacks.

She added: "The last few days in this weather have been a nightmare for me. My asthma is bad anyway; I have to use a nebuliser to keep attacks under control but, on Saturday, I thought I was going to die."

Why it is deadly - and how it has changed

By Steve Connor

Modern smog is caused by a combination of traffic exhaust fumes and unusual weather conditions that prevent pollution from being blown away, causing a build-up to levels that can affect breathing.

Bright sunlight causes an additional problem by reacting with the nitrogen oxides from car exhausts to produce ozone, a highly reactive gas that at ground level can irritate the lungs, especially in people with asthma.

Smog used to mean "smoky fog" and was a term first used more than a hundred years ago to describe the pea-soupers of Dickensian London. But since the Clean Air Act 50 years ago, the chemical make-up of smog has changed and the term now applies to a build-up of nitrogen oxides and ozone from burning petrol rather than smoke particles from coal fires.

High pressure over Britain in recent days has caused a temperature inversion in the atmosphere over much of the country.

An inversion is when the temperature increases rather than decreases the higher you are from ground level. Normally, temperatures decrease as you rise above the ground. Such temperature inversions result in a mass of colder air sitting on the layer of air nearest to the ground, which acts like a lid keeping the pollutants in smog from dispersing.

Another problem in modern smog is the presence of tiny particles, or particulate matter known as PM10s, which are so small they can be breathed deep into the lungs. Again, exhaust fumes are a major cause of PM10s.

Smog can affect rural areas as well as towns and cities and the South-east is particularly susceptible to smog that blows over from major urban centres on the Continent.