Exposure to London's 1952 smog linked to increased incidence of childhood, adult asthma

London's Great Smog of 1952 resulted in thousands of premature deaths and even more people becoming ill. The five December days the smog lasted may have also resulted in thousands more cases of childhood and adult asthma, according to research published in the American Thoracic Society's American Journal of Respiratory and Critical Care Medicine.

"We were interested in seeing if this event affected early childhood health and, if so, what the long-term health consequences were," said co-investigator Prashant Bharadwaj, PhD, associate professor of economics at the University of California, San Diego, who was surprised to learn of the existence of health data from the 1940s and 50s that made the research possible. "We now know that more than 60 years later, the Great Smog likely still affects some people's health."

In "Early Life Exposure to the Great Smog of 1952 and the Development of Asthma," Dr. Bharadwaj and his colleagues write that the Great Smog presents a "natural experiment" because the smog was intense, "exceeding current regulations and guidelines by a factor of 5 to 23"; localized to a major city; and unanticipated. "Because the smog was unexpected, residents likely didn't leave the city," said Dr. Bharadwaj. If those with the resources and ability had left, he added, the analysis would have been biased.

The researchers analyzed 2,916 responses to a life history survey that is part of the English Longitudinal Study on Aging. Among other health questions, the survey asked participants if they had asthma as a child (up to age 15) or asthma as an adult.

The researchers compared the responses of those who were exposed to the Great Smog in utero or in early childhood, with those born between 1945 and 1955 who lived outside of London during the Great Smog or lived in London but were not exposed to the smog in utero or in their first years of life.

The results showed that exposure to the Great Smog in the first year of life was associated with a statistically significant 19.8 percent increased incident of childhood asthma. Researchers said they found a non-significant, but similar trend between exposure to the smog in the first year of life and adult asthma (a 9.5 percent increase) and in utero exposure and childhood asthma (a 7.9 percent increase).

The authors note that there are a growing number of studies examining the relationship between early childhood exposure to air pollutants and the development of asthma. These studies, they say, can only determine an association, not a cause-and-effect relationship, because there may be confounding factors that are overlooked or not fully accounted for in the analysis.

Given that there is no evidence of another event simultaneous with the Great Smog that might affect asthma incidence, they add, their study overcomes the issue of confounding and "suggests a strong possibility of a causal link between early childhood exposure to air pollution and the later development of asthma."

The study has implications for other countries and cities today with high levels of air pollution. In recent years, the authors write, Beijing has experienced the highest levels of air pollution ever recorded.

"We have to recognize that air pollution can have long-term effects, especially on kids," Dr. Bharadwaj said. "Children don't get to make decisions about where they live so governments need to take them into account when they make decisions about reducing the burden of air pollution caused by increasing development."