It's summer, it is sunny and we have smog. The word was coined at the turn of the century to describe the acidic smoke-plus-fog that regularly blighted the London winter. Now summertime smog is that all- pervading, dirty haze that hangs over the sweltering streets of our major cities, making it difficult to breathe and move around in the heat
Summer smog is a result of a complex cocktail of noxious chemicals emitted from vehicle exhausts. On still, sunny days these react together in bright sunlight to produce yet more irritable substances, notably ozone, which constricts the lungs, causing coughing and other breathing difficulties.
Ozone at high altitudes is both natural and beneficial, acting as a shield against harmful ultraviolet radiation from space. But near the ground it is a pollutant that can cause serious health problems in high concentrations.
As the love affair with the car has blossomed (car use is expected to double in the next 30 years), smog has become a regular feature of our summers. In the past week, few parts of Britain have escaped poor air quality. London and other cities felt it most, primarily because of rising nitrogen dioxide from exhaust fumes, but rural areas also had increasing levels of ozone, building up as a result of vehicle emissions drifting across from many miles away.
The Government's response is to monitor the most noxious chemicals, offer advice on how the public can reduce its own contribution to the problem and to work within a European ''framework'' of soft controls aimed at reducing emissions. All new cars have to be fitted with catalytic converters, which eliminate some of the most harmful substances, but the only measures to reduce the amount of traffic have come in the form of voluntary advice.
Friends of the Earth and other environmental pressure groups have been quick to criticise the response. They believe draconian rules, such as reducing motorway speed limits, similar to those imposed on German motorists during the worst pollution incidents, are now needed to get on top of the problem.
Other campaign calls include the introduction of further parking restrictions in urban areas, and outright bans on traffic in certain city streets.
Although practically everyone, government included, is agreed that something has to be done to persuade people to use their cars less, the same consensus has not been reached on the health effects of summer smog. Although both ozone and nitrogen dioxide are known to be toxic in concentrations well above those seen in recent pollution episodes, there is little if any evidence that they can cause serious, long-term illness at the levels seen in even the worst examples of summer smog.
An international panel of health experts found that even when levels of ozone rise to 100 parts per billion - "poor" air quality - the resulting inflammation in the lungs of some susceptible people is unlikely to have any long-lasting effect. Some scientists believe the link with asthma is equally tenuous.
Although smog in summer feels unpleasant it may be have more to do with the heat of working in a city full of traffic jams than the actual health effects of noxious exhaust fumes. Ultimately, our concerns about summer smog may have more to do with the realisation that our love affair with the car ended some time ago.