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Nobody realised what was happening at first. There were no bodies on
the streets when the great London smog of December 1952 engulfed the capital.
‘One of the first indications was that undertakers were running out of coffins
and florists out of flowers,’ remembers Robert Waller, who was then a medical
researcher at St Bartholomew’s Hospital in London. ‘Only later was it realised
that the number of deaths per day during the smog was three or four times
normal. Nothing like this had ever happened before.’

The smog lasted five days and killed more than 4000 people, according
to Hugh Beaver, chairman of the advisory council of the Department of Scientific
and Industrial Research, in his official report, published in November 1954.
At its height, people had been dying faster than in the cholera epidemic
a century earlier. Beaver commented that it ‘must in truth be a supreme
example of the way in which a metropolis of eight-and-a-quarter million
people can experience a disaster of this size without being conscious all
the while of its occurrence.’

Winter smogs, in which smoke and sulphur dioxide from the city’s chimneys
accumulated in the foggy air, had been a feature of London life since at
least the 17th century. It was then that the diarist John Evelyn described
a ‘hellish and dismall cloud of sea-coal’ over the capital, and King James
I complained that smoke was corroding St Paul’s Cathedral. Smogs were described
by Charles Dickens in Bleak House, and painted by the Impressionist Claude
Monet. They became especially frequent at the end of the 19th century. Smog
helped create the worlds of Sherlock Holmes and Jack the Ripper. Early this
century, the frequency of smogs fell as the capital spread out and became
less congested, but they returned with a vengeance in the years after the
Second World War, possibly exacerbated by the introduction of ‘nutty slack’,
a low-grade coal with a high ash content, to cope with fuel shortages.

Other urban areas suffered serious smogs too, in the first half of this
century, from Glasgow and Manchester to New York and the Meuse valley in
Belgium. But the London smog of 1952 was unique, says Peter Brimblecombe,
a lecturer in the environmental sciences department at the University of
East Anglia, whose book The Big Smoke is the definitive history of London’s
air pollution: ‘The air virtually became stationary over London for a week.’

The climatic trigger for this event was the formation of a static layer
of cooler air close to the ground as the night-time temperature dropped.
This is known as a temperature inversion because, usually, air close to
the ground is warmer than the air above it, and rises. Such inversions are
frequent on winter nights after the ground has cooled down so much that
it begins to chill the air closest to it, often causing mist to form as
water vapour precipitates on dust particles. Normally the morning sun swiftly
breaks through the mist and heats the ground, which warms the air above
it, breaking the inversion. But in December 1952, the accumulation of smoke
close to the ground was so great that the sun never broke through, and the
air stayed cold and static.

‘The smog stayed stewing away as more and more pollution entered it,’
says Waller. The dense yellowy-black sulphurous ‘peasouper’, named after
its colour as well as its density, persisted for an unprecedented five days
from 5 to 10 December, before winds from the west blew it down the Thames
estuary and out into the North Sea. At one point, the smog extended for
30 kilometres around London, reducing visibility between one and five metres.

Since the great smog, researchers have attempted to piece together its
chemistry. Hanging in the air were thousands of tonnes of black soot, sticky
particles of tar and gaseous sulphur dioxide, all of which came mostly from
coal burnt in domestic hearths. ‘The water from the fog condensed around
the soot and particles, and the sulphur dioxide reacted inside these foggy,
sooty droplets to form dilute sulphuric acid creating, in effect, a very
intense form of acid rain,’ says Brimblecombe. The formation of acid may
have been catalysed by other pollutants, such as iron, he believes. While
nobody measured the acidity of the smog at the time, its pH was certainly
less than 2, says Waller, and was probably about 1.6 – as acid as the electrolyte
in a car battery or dilute sulphuric acid on a laboratory shelf.

From the middle of the 19th century onwards, says Waller, doctors had
suspected that the smogs were killing people – particularly the very young,
the bronchitic and the old. On a number of occasions they had measured the
extra deaths during Victorian smogs at several hundred. But the doctors
lacked proof that pollution was to blame. It was difficult to distinguish
the smog effect from the widespread occurrence of infectious lung diseases
at that time and the increased death rate from hypothermia, which is still
a feature of Britain today.

PINPOINTING THE KILLER

But by 1952, infectious diseases had declined and, unusually, the smog
of that year was not accompanied by bitter cold – the overnight temperatures
were below freezing, but not exceptionally low for the time of year, according
to the records of Meteorological Office. ‘For the first time, it was possible
to pick out the specific effect of pollution,’ says Waller. But for that
chance set of circumstances, the case that smog kills might not have been
established. Subsequent research by Waller and a team at St Bartholomew’s
convinced them that it was not so much the original pollutants – the soot
and sulphur dioxide – that killed, but the acidity of the air. Breathing
in the acid aerosol irritated the bronchial tubes, which produced large
amounts of mucus and became inflamed. Bronchitics and people with other
respiratory problems such as asthma, wheezed to their deaths in their beds.
They choked on their mucus or died of a heart attack as they fought for
breath. Though the majority were old, the death rate among young children
doubled during the week of the smog and it trebled for those aged between
45 and 64.

Every part of London suffered. As far out as Croydon, about 20 kilometres
south of the centre, hundreds died and people attending council offices
to register deaths were told to come back the next day because staff could
not cope with the workload. But some of the highest death tolls were in
the slums of districts such as Camberwell, which lies just south of the
River Thames. There, the usual death rate of five people a day climbed to
21 on the second day of the smog, and to 50 on the third. Animals succumbed
too. A dozen prize cattle died at the annual Smithfield Show west London.
It was later discovered that most of the dead beasts had clean bedding,
while those with soiled bedding fared better, presumably because ammonia
emanating from it helped to neutralise the acid in the air.

Even after health minister Iain Macleod revealed the extent of the disaster,
some people continued to believe that smogs were an inescapable fact of
life. As The Times put it: ‘Fogs are not parasites of coal fires. They are
ancient Britons.’ The local government minister, Harold Macmillan, grumbled
to the Cabinet: ‘Today everybody expects the government to solve every problem.
It is a symptom of the welfare state. For some reason or another ‘smog’
has captured the imagination of the press and people . . . I would suggest
we form a committee. We cannot do very much, but we can seem to be very
busy – and that is half the battle nowadays.’

He was wrong. Just as the cholera epidemic in London in 1866 triggered
a revolution in public health provisions, especially in water supplies,
so the great London smog of 1952 galvanised politicians to clean up the
nation’s air. Jarred by a popular private member’s bill calling for curbs
on emissions of pollutants, the government in 1956 passed the Clean Air
Act. This authorised local councils to set up smokeless zones and make grants
to householders to convert their homes from traditional coal fires to heaters
fuelled by gas, oil, smokeless coal or electricity.

In January of that year, around 1000 people had died in a smog and,
with the legislation passed, a further thousand perished in December 1957.
Progress, says Waller, was necessarily slow. For example, new power stations
had to be built to meet the surge in demand from people who wanted to
abandon fossil fuels altogether, preferring instead the convenience of electric
heating.

‘We envisaged that the task might take 20 to 30 years,’ remembers Waller.
Thankfully for the capital’s health, however, a sharp cut in gas prices
in the 1960s, as abundant supplies from beneath the North Sea became available,
spurred householders into forsaking coal fires forever. The final major
London smog, in which a further thousand people took their last, choking
breaths, occurred in December 1962.

Brimblecombe wonders how critical, in reality, the Clean Air Act was
to the change. ‘A lot of the things would have happened without it,’ he
says, ‘because already there was a shift away from the coal fire. Many people
wanted all-electric houses. The legislation reinforced a change that was
already under way.’

Nowadays, with the gritty coal smoke largely gone from London, it is
the finer particles from diesel exhaust fumes, belching from the backs of
buses, lorries and taxis, that are the the city’s main source of smoke pollution.
Whenever temperature inversions strike, this smoke accumulates in the air,
along with the nitrogen oxides (nitric oxide and nitrogen dioxide, known
collectively as NOx) produced by car exhausts. Nitrogen dioxide, a serious
lung irritant that has now replaced sulphur dioxide as the most feared urban
pollutant, was the main ingredient in the murky smog that descended on the
capital just before last Christmas. Levels of the gas were the highest ever
recorded in London, prompting The Times, in a moment of historical ignorance,
to describe the event as London’s ‘worst ever smog’.

But for most of the world, the word ‘smog’ no longer conjures up images
of smoke and winter, but rather vehicle fumes and summer. These summer smogs
feed off the large amounts of nitrogen compounds emitted all year round
into the air of modern cities, mostly by vehicles. The smogs are driven
by sunlight, which encourages a wide range of reactions that create secondary
pollutants. Temperature inversions exacerbate them.

The archetype is the Los Angeles photochemical smog, first described
in 1948 and now plaguing cities from Athens to Mexico City. While no photochemical
smog is yet known to have caused carnage on the scale of London’s 1952 peasouper,
this may be for want of good data, as was the case in London for a century.
In any event, their chemical cocktail is potentially at least as dangerous.
Photochemical smogs too can cause respiratory problems, including asthma,
which is showing a rapid increase in incidence in many Western countries.
Moreover, these smogs can spread far beyond city centres into remote rural
areas.

The main inputs are NOx and a class of hydrocarbons known as volatile
organic compounds (VOCs). Like NOx, VOCs are spewed out in large quantities
from car exhausts. They also enter the atmosphere through the evaporation
of many widely used products, including cleaning solvents, glues and petrol.
Annual emissions of VOCs in Britain are currently around 2.5 million tonnes,
and those of NOx are approaching 3 million tonnes.

WHEEZING IN SUMMER

VOCs, like the nitrogen oxides, are health hazards in themselves, causing
eye irritation and coughing. Benzene, a VOC found in diesel exhaust, is
a known carcinogen. Most notable among the range of secondary pollutants
they produce is ozone. While this gas is present naturally in the stratosphere,
where it acts as a shield against harmful ultraviolet radiation, it is not
at all beneficial when it accumulates at lower altitudes. At ground level,
ozone makes breathing difficult, damages trees, reduces the yield of crops,
and corrodes a wide range of materials, including metals, rubber and fabrics.
The gas also speeds up the formation of acid rain and acid fog and thus,
in some situations, can contribute to a lethal combination of the old and
new types of smog.

Background levels of ozone in the lower atmosphere are typically 30
parts per billion, twice the pre-industrial level. The World Health Organization
sets a recommended limit of 100 ppb averaged over one hour and 60 ppb over
eight hours. During 1990, parts of southeast England exceeded 100 ppb of
ozone for more than 45 hours. During major smogs, when light winds and strong
sunlight allow the chemicals to ‘stew’, the concentration can soar to 200
ppb or more. Such events occur only once every few years in Britain, but
in hotter climates, they can last for weeks at a time. In 1990, ozone concentrations
in Mexico City exceeded WHO guidelines on 310 days. This year, ozone levels
in the city reached 398 ppb. In response – and with doctors reporting record
incidences of asthma, pneumonia and bronchitis – the government shut schools,
ordered vehicles off the streets and closed factories. In the US throughout
the 1980s, health limits for ozone were exceeded during summer months in
dozens of cities.

The chemistry of modern smogs can be immensely complex, with the photochemical
degradation of a single molecule of a particular VOC perhaps involving 100
or more separate reactions, many of which can be catalysed by the products
of other reactions. This makes the impact of reducing emissions very hard
to predict. For instance, perhaps the most important and characteristic
process is the oxidation of nitric oxide (NO) from vehicle exhausts to nitrogen
dioxide (NO₂) and its subsequent conversion, through reactions
with the products of the photochemical breakdown of VOCs, to ozone (O₃).
But nitric oxide can also destroy ozone, which is one reason why ozone concentrations
are often highest in the suburbs, away from the main pollution sources in
city centres.

PERSISTENT POLLUTION

Another reason is that the reactions that create ozone take an hour
or more to get going and may still be in full flow in a parcel of air that
has travelled for 5 or even 10 hours away from the original source of the
pollutants. The photochemical reactions cease at night, though it is now
clear that a different range of reactions can take place in smoggy air during
darkness. The next morning, the ozone and other reactive molecules are generally
still present to resume their photochemical progress.

Thus ozone smogs can travel for hundreds of miles and last for many
days, gaining new impetus whenever they pass over a new source of pollutants.
Unlike the old static smogs, they frequently cross international boundaries.
In 1976, one smog that began in central Europe was tracked several days
later crossing the shores of Ireland from the Atlantic Ocean. Britain’s
Department of the Environment says that pollution from Continental Europe
‘plays a significant role in UK ozone episodes’.

To complicate matters further, a range of naturally occurring VOCs will
also react with nitrogen dioxide to create ozone and other compounds; they
could perhaps take the place of emissions of synthetic VOCs as these are
reduced. During the US election in 1980, Ronald Reagan was widely derided
for claiming that trees caused pollution. But the claim had a basis in fact.
It followed the discovery that trees produce large amounts of VOCs. Under
typical urban conditions, the ozone-forming potential of these VOCs is equal
to that of synthetic ones. Confusingly, however, in rural areas, hydrocarbons
from trees generally destroy ozone. So, anyone destroying forests in the
name of clean air is likely to be proved mistaken.

The visible part of a photochemical fog – or rather, the particles that
reduce visibility in cities such as Los Angeles – consists of a wide range
of tiny particles, most of them less than 2 micrometres in diameter. In
a typical Los Angeles smog, 40 per cent of the particles are soot from burnt
oil and diesel fuel. These tiny particles can penetrate deep into lungs,
causing irritation, and some are known to be cancer agents. Other common
fine particles include traces of metals such as iron, as well as sulphates
and nitrates, which can contribute to the acidification of fog droplets.
In fact, a photochemical smog can sometimes be as acidic as an old London
peasouper. A pH as low as 1.7 has been recorded in Pasadena, California.

To meet the growing threat, southern California is planning measures
to reduce VOC and NO_x emissions that will have an effect on almost every
aspect of modern living. They range from controls on lighter fuel, motor
mowers and barbecue charcoal to switching the state’s vehicle fleet from
burning petrol to methanol by 2010. Industrial emissions are to be controlled
from next year by opening up a free market in ‘pollution permits’, to be
issued to companies according to their current emissions. The number of
permits will be reduced year by year, but companies will be able to buy
and sell their permits at the market rate. This is intended to ensure, in
theory at least, that investment on plant that will reduce emissions is
made in the most cost-effective manner, with minimum bureaucracy.

European cities such as Athens and Rome have introduced systems to limit
traffic during smogs. One scheme allows only cars with odd registration
numbers on the streets on half the days of the week, and only those with
even numbers during the rest of the week. Similar efforts are now underway
in Mexico City, the world’s most populous city, with 20 million inhabitants
and 3 million vehicles. Smog there is worsened by frequent temperature inversions,
and in its thin atmosphere – the city is more than 2000 metres above sea
level – the oxygen pressure is 20 per cent less than it is at sea level,
so fuels burn less efficiently.

But the city’s troubles are unlikely to be eased by a recent proposal,
apparently under serious consideration by the mayor. The idea is to install
100 giant hot-air generators across the metropolis in an effort to blast
a hole through the temperature inversion during smog episodes, so allowing
the pollution to escape. It sounds harebrained, would cost an estimated
$100 million, and its generators would themselves cause considerable additional
pollution. Like a scheme briefly proposed after the great London smog to
use sound waves to clear the smoke, it fails to attack the root cause: the
source of pollution.

A better approach would be that adopted by the Japanese. During the
1970s, Tokyo, then the world’s largest city, became a byword for nitrogen
and ozone smogs in both winter and summer. But a concerted effort, including
one of the world’s first programmes to fit catalytic converters to cars,
has begun to improve the quality of the urban air. It has also given Japanese
companies the lead in a new worldwide industry.

Mexico City, meanwhile, risks a pollution disaster on the scale of London’s
40 years ago. And, as the giant cities of the Third World continue to grow,
and the number of cars on their streets increase the risks to their citizens
can only rise.

Fred Pearce will present ‘The Great London Smog’ on BBC Radio 4, at
4.30 pm on 27 December. Peter Brimblecombe’s book The Big Smoke was published
in 1987 by Methuen.