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.
The city's pollution changes seasonally: in summer a "low inversion layer" traps pollutants, bathing the city in a dirty haze; during the sub-zero winter, when burning coal provides almost all the power and heat, one can almost taste the coal-dust in the air. In spring, dust storms blow in from the Gobi desert, and the bicycle lanes swarm with women whose faces are covered with gauze scarves. Motor vehicles have tripled in four years, to 900,000. Lead-free petrol is not on offer, and many vehicles and public buses belch out clouds of black exhaust.
In 1990, city centre sulphur dioxide readings exceeded WHO levels by a factor of two, while particulate matter levels exceeded limits by a factor of more than four - higher in the winter. Summer ozone concentrations are routinely higher than WHO recommended levels. Some Western embassies are considering issuing staff with personal monitoring badges to measure the pollution levels they endure. Chinese entrepreneurs have spotted a market opportunity: three "oxygen bars" have opened this year, where one can sit and rent an oxygen mask to recover.
Last year the government moved 16 factories and workshops out of Peking. But coal combustion will, for the forseeable future, continue to provide three-quarters of China's rising energy demands. Teresa Poole
The Los Angeles basin is the most air-polluted area in America. Each day a meteorologist at the county Air Pollution Control District estimates probable atmospheric pollution for the local media to broadcast. The level of ozone reached 161 on Saturday; 131 is the federal standard at which healthy people are advised to reduce physical activity.
For two decades, California has forced the introduction of anti-smog technologies, from catalytic converters in the Seventies to prototypes for mass-produced electric cars in the Nineties. But despite strict domestic, industrial and car emissions laws, the grindingly slow development of a metro and an agreement by the motor industry to guarantee that 2 per cent of new cars sold by 1988 will be electric, or emission-free, the problem is still severe. There are incentives for people to share transport: they drive on a clear lane on freeways.
Every new car in California must be equipped with a catalytic convertor and older cars must pass an annual smog test. Enforcement officers with sensory devices, "Smog Dogs", beam infrared light across lanes of traffic, instantly recording the levels of carbon monoxide and hydrocarbons from the tailpipes of passing vehicles.
For 25 years California has slashed pollution by reducing oil refinery emissions, installing special nozzles on gasoline pumps and exhaust controls on cars, and restricting the sale of oil-based paint. Proposed new federal laws include a 30 per cent reduction in emissions from solvents used to apply pesticides, and requirements for companies owning fleets of cars or trucks to convert some vehicles to electricity or natural gas.
The approach of the year's first tropical storm threw a yellow haze over Hong Kong island last weekend. Yet because the colony is so polluted, the deterioration in air quality was classified as "only slightly higher than usual". Since 1989 a raft of controls and massive relocation of industry to China has reduced sulphur dioxide levels. However, dust and grit emissions rose as Hong Kong's construction frenzy intensified and vehicle numbers increased. In 1991 unleaded petrol was introduced, the following year stringent emission standards were enforced for new vehicles and from this year all new vehicles must have catalytic converters. About 60 to 70 per cent of heavy road users are in diesel-powered vehicles, many of which are buses and mini-buses. The government is trying to introduce cleaner diesel fuel and encourage lighter vehicle owners to move to petrol. But the tax regime still favours diesel over petrol. Steve Vines
In summer 1987, 800 people died from a combination of a freak heatwave and inhalation of poisonous fumes. Now, the municipal authorities have introduced drastic traffic restrictions. Since April private cars have been banned from a mile-square area of the city centre. The authorities plan to extend the experiment to other parts of Athens They are also expanding the metro system. With 3 million vehicles in the region, an area surrounded by mountains and trapped in its own pollution, public support for such steps is strong. Tony Barber
Cars are not a source of air pollution in Singapore. Most motor vehicles are scrapped after 10 years - owners who want to keep them for another 10 years have to pay a premium costing from pounds 15,000. And there is a quota for the number of new motor vehicles: annual growth is limited to around 3 per cent of total vehicles, currently 629,000. The main source of air pollution is external - for the past few years it has been forest fires in neighbouring Indonesia - so the problem takes on international dimensions. The forest fires are the result of villagers burning off rice husks or increasing acreage for cultivation by burning the forests; and in recent years they have created a blanket of smog over Singapore, from August until the north-east monsoon starts blowing in October/ November. The Indonesian forest fires have caused many in Singapore to worry about their neighbour's plans to build nuclear plants.
This was the first European city to make face masks popular for cyclists and pedestrians, and one of the first to introduce anti-pollution traffic controls. Emissions from cars, metallurgy factories, wood-burning and any number of organic pollutants such as solvents and paint produce a heady cocktail of poison gases when the temperature rises and the wind drops. One study showed that on hot-weather days, deaths increase from the daily average of 32 to 35 or 36.
The ozone level, along with other gases such as nitrogen dioxide, has been rising at 0.5 to 2 per cent per year, forcing the city authorities to set up a monitoring service which alerts them whenever the ozone level rises above 180 microgrammes per cubic metre. When that threshold was crossed 10 days ago, the city immediately urged residents to leave their cars at home and advised pensioners and parents with young children to stay indoors during the early afternoon. Naples and Genoa introduced a two-day ban on traffic in the centre. Milan has introduced spot checks on exhaust pipes. But some believe only an exhaustive restructuring of the city's industries can reduce air pollution.
Four million cars, 30,000 industrial plants and a high plateau location have made Mexico City probably the world's most-polluted city. In 1993, ozone levels exceeded WHO "safe" levels on 334 days. Sore throats, stinging eyes, splitting headaches, even asthma attacks are accepted as the norm. Added to exhaust and industrial pollution is the prevalent dust from dried urine and faeces, mainly from dogs, rats and humans.
The government has taken various measures, some ostensibly drastic, in recent years. The city claims to have spent $5bn on clean air programmes since 1988. In 1990, the "hoy no circula" (today you don't drive) campaign was launched, taking one-fifth of the city's cars off the road on any given day, according to the last digit of the number plate. True to form, however, Mexicans simply wheeled out their second or third cars, adding 300,000 vehicles to the road within weeks. Others simply switched number plates.
In 1991, the federal government ordered all new cars to have catalytic converters and pushed the use of lead-free petrol. That left the hundreds of thousands of Fifties, Sixties and Seventies gas-guzzling Chevvies, Ford Galaxies etc, which Mexicans somehow manage to keep on the road. Mandatory twice-yearly exhaust checks were introduced, with a centralised computer system used to curtail the use of bribes.
Last year, a 200-person "Green Police" force was set up to watch for and fine drivers of polluting vehicles but they, too, poorly paid, have had a tendency to settle for pa'chesco (a tip) to turn a blind eye. Phil Davison