Mortality did not return to normal levels for several months after the episode. The strong immediate health response to the episode is evident in the coinciding sharp increase in mortality; however, the elevated mortality in the months after the smog requires a more detailed analysis. An initial government report (U.K. Ministry of Health 1954) proposed the hypothesis that influenza was responsible for the elevated mortality in the months that followed the episode. The weekly number of excess deaths in Greater London (the number of deaths exceeding those during the same time period the previous year) peaked at about 4,500 for the week ending 13 December 1952. Total mortality rates were about 80% higher than the previous year for December 1952 and were 50 and 40% higher, respectively, for January and February 1953. From December 1952 through March 1953, there were over 13,500 more deaths than normal. A fraction of these likely resulted from air pollution and a fraction from influenza.
The exact numbers of influenza-related and air pollution-related deaths are unknown and continue to generate debate (Stone 2002). Estimates of influenza deaths generated through multiple approaches contradict the influenza theory, indicating that far more people died from air pollution than originally believed. Estimates of the number of influenza deaths were constructed using observations from general practice of medicine in the London area and the control group of a vaccine study conducted at the time. The reconstructed estimates of influenza mortality reveal that only a portion of the excess deaths after the smog can be attributed to influenza.
Estimates of Influenza-Related Mortality
The number of deaths from influenza was estimated using two approaches: observations from a general medical practice and data from a vaccine trial. Observations from a general medical practice of about 6,000 persons in Greater London from 1949 through 1968 provided an estimated case-fatality rate of 0.2% (Fry 1969). For the years that had an influenza epidemic, Fry estimated that the percentage of patients who contracted influenza ranged from 3% to 17% and averaged 8% with a median of 6.5%. For the 1953 year, 6% of the patients had influenza. This supports other documents that reported a mild influenza epidemic during that time (U.K. Ministry of Health 1953, 1956).
The Medical Research Council Committee on Clinical Trials of Influenza Vaccine organized a clinical trial of an influenza vaccine with 12,710 volunteers in London and other cities (Committee on Clinical Trials of Influenza Vaccine 1953). The majority of volunteers were inoculated between 26 November and 5 December 1952, with some inoculations on 12 December 1952. Follow-up continued through 31 March 1953. The attack rate in the control group was 4.9% during the winter of 1952-1953. The original researchers divided areas into three regions based on their mortality rates because high mortality areas were assumed to have higher incidence of influenza. London was in the highest incidence category, with an attack rate of 6.61% in the control group of 1,181 volunteers. This value is similar to the incidence rate observed for the winter of 1953 for the general medical practice.
The original government report on the health effects of the episode (U.K. Ministry of Health 1954) proposed that an influenza epidemic caused the elevated mortality, although those authors recognized that some deaths in the months after the smog could be related to the air pollution from the episode. Excess deaths from this time through the end of March number more than 8,000. The report provides estimates of the number of deaths attributable to the smog using several different approaches, which do not include any deaths after 20 December 1952. The U.K. Ministry of Health estimated that 5,655 people died from influenza in the first 3 months of 1953 (U.K. Ministry of Health 1956). Another source reported 5,647 influenza deaths for the first 3 months of 1953 in England and Wales (World Health Organization 1953).
To better understand the role of influenza and pollution in the deaths that took place in the months after the extreme episode, an estimate of the number of influenza-related deaths was constructed using information from the observations of a general medical practice and the vaccine trial. The population of Greater London at this time was approximately 8.6 million, based on the 1952 census (U.K. General Register Office 1951). Because early government reports attributed no deaths after 20 December 1952 to pollution and because mortality rates remained elevated for several months, especially for January and February 1953, we calculated the number of excess deaths from 21 December 1952 through 28 February 1953. During this period, 8,275 more deaths occurred than expected based on the previous year, which did not have an influenza epidemic or extreme air pollution events.
Sensitivity Analysis
The incidence rates generated through observations in a general medical practice are likely to be lower than the true incidence because some patients with influenza may not have consulted their physician. It is plausible that the underestimation is large if a significant number of patients with influenza did not consult their primary physician and he did not learn of their condition. In contrast, the case-fatality rate could be an overestimation, assuming the most ill patients sought medical care and few of the patients with influenza that did not receive medical care died. The attack rates from the vaccine control study (Committee on Clinical Trials of Influenza Vaccine 1953) could be lower than those in the general population because of the selection process of volunteers for the study.
Figure 2 provides the number of influenza deaths from 21 December 1952 through 28 February 1953 if the estimate generated using the 1953 winter rate from the general medical practice is too low by varying degrees. Results show that the estimated case-fatality and incidence rates for influenza would have to be drastically understated for influenza to account for the excess mortality after the smog. The most severe epidemic observed during the 20-year period of the general medical practice observations took place in 1957. If influenza after the pollution episode were twice that severe, many deaths still remain unexplained. If both the 1953 incidence rate and the case-fatality rate were underestimated by 100%, the corrected value of influenza deaths would be approximately 4,000. Only an extremely severe influenza epidemic could account for the majority of the excess deaths for this period. Such an epidemic would be along the order of twice the normal case-fatality rate and quadruple the incidence observed in 1953. These results reveal that even if the estimates of the number of influenza-related deaths, as presented in Figure 1, are substantially lower than the true values, a larger number of deaths remain unexplained by influenza.
Figure 1. Estimated number of influenza deaths in Greater London, 21 December 1952 through 28 February 1953. Methods of estimation are as follows: A, control group from influenza vaccine study; B, control group from influenza vaccine study, for areas of high mortality; C, observations from a general medical practice, using the average incidence rate of years with influenza epidemics; D, observations from a general medical practice, using the incidence rate for the 1953 winter; E, government estimate for January to through March 1953. Data from the Committee on Clinical Trials of Influenza Vaccine (1953), Fry (1969), and the U.K. Ministry of Health (1953)
.
Figure 2. Sensitivity analysis for number of influenza deaths in Greater London, 21 December 1952 through 28 February 1953. Methods of sensitivity analysis estimates are as follows: A, case-fatality rate and incidence from the observations of a general medical practice for the 1953 winter both increased by 25%; B, case-fatality rate and incidence from the observations of a general medical practice for the 1953 winter both increased by 50%; C, case-fatality rate and incidence from the observations of a general medical practice for the 1953 winter both increased by 100%; D, incidence of the most severe influenza epidemic observed for the general medical practice from 1949 through 1968 (17% for 1957 epidemic); E, incidence twice that of the 1957 epidemic (34%); F, case-fatality doubled and incidence four times higher than from the observations of a general medical practice for the 1953 winter.
The relationship between the Great Smog and the Sulphur Dioxide
Figure 2.1 shows the average smoke and sulphur dioxide levels for 12 London sites and the relationship with deaths recorded during the smog period in December 1952. The peak in the number of deaths coincided with the peak in both smoke and sulphur dioxide pollution levels.
Figure 2.1: The Relationship Between Smoke And Sulphur Dioxide Pollution And Deaths
During TheGreat London Smog, December 1952, Source: Wilkins, 1954
Air Pollution During The Great London Smog, 1952
Smoke and sulphur dioxide pollution was monitored at various sites in London at the time of the December 1952 smog. The daily average measurements for 10 of these sites are given in Table 2.1.
Table 2.1: Pollution Concentrations, London, December 1951 & 1952, Source: Wilkins, 1954
Table 2.1 and Figure 2.1 show the dramatic increase in smoke and sulphur dioxide levels during the smog which occurred between 5th and 9th December. The mean levels for December 1951, when there was no fog, are given in Table 2.1 for comparison.
The Clean Air Acts, 1956 & 1968
The Government could not ignore the Great London Smog. The Clean Air Act was eventually introduced in 1956 following the Beaver Committee Report. This Act aimed to control domestic sources of smoke pollution by introducing smokeless zones. In these areas, smokeless fuels had to be burnt. The Clean Air Act focussed on reducing smoke pollution but the measures taken actually helped to reduce sulphur dioxide levels at the same time. Air pollution in cities dramatically reduced in the following ways:
- domestic emissions reduced because of smoke control areas;
- electricity and gas useage increased and the use of solid fuels decreased;
- cleaner coals were burnt which had a lower sulphur content;
- use of tall chimney stacks on power stations;
- relocation of power stations to more rural areas;
- continuing decline in heavy industry.
Tall Chimneys
The Clean Air Act of 1968 brought in the basic principle for the use of tall chimneys for industries burning coal, liquid or gaseous fuels. At the time of this legislation it was recognised that smoke pollution could be controlled but that sulphur dioxide removal was generally impracticable. Hence, the higher the chimney, the better the dispersal of the air pollution.
Urban Air Quality After The Clean Air Acts
Urban air quality improved following the Clean Air Acts. In particular, the smoke, grit and dust that arose from industrial and domestic sources due to coal burning had been controlled through the introduction of smokeless zones and the controls imposed on industries to reduce their particulate emissions.
Levels of smoke and sulphur dioxide fell considerably in towns and cities during the 1960s and 1970s and levelled off during the 1980s. The success in reducing urban air pollution and the smogs of the past was due to the introduction of the legislation, particularly the Clean Air Acts. Figures for smoke and sulphur dioxide for Manchester for the winters (October - March) between 1960 and 1990 are shown in Table 2.2.
Table 2.2. Winter Smoke And Sulphur Dioxide Concentrations, Manchester 1960-1990, Source: Lewis, 1995
Conclusion
The Great Smog from 1952 is considered the worst event in the and the most significant in terms of its impact on environmental research, government regulation, and public awareness of the relationship between air quality and health. Although London was accustomed to thick fogs, this one was denser and longer lasting than any previously seen. Visibility was reduced to a few yards ("It's like you were blind", commented one observer, making driving difficult or impossible. Public transport ground to a halt – apart from the – and the ambulance service stopped running, forcing the sick to make their own way to hospital. The smog even seeped indoors, resulting in the cancellation or abandonment of concerts and film screenings (as visibility fell in large enclosed spaces, and stages and screens became harder to see from the seats). Outdoor sports events were also affected.
Initially, there was no great panic, as London was renowned for its fog. In the weeks that followed, however, statistics compiled by medical services found that the fog had killed 4,000 people. Most of the victims were very young, elderly, or had pre-existing respiratory problems. Deaths in most cases were due to from , and due to mechanical obstruction of the air passages by arising from lung infections caused by the smog.The lung infections were mainly or acute superimposed upon chronic . A total of 12,000 people are believed to have died in the weeks and months that followed.
Nelson’s Column during the 1952 smog
Bibliography:
(Environmental Health Perpectives)
(Atmosphere, Climate and Environment Information Programme)
http://en.wikipedia.org/wiki/Great_Smog_of_1952