<P> Smog is a serious problem in many cities and continues to harm human health . Ground - level ozone, sulphur dioxide, nitrogen dioxide and carbon monoxide are especially harmful for senior citizens, children, and people with heart and lung conditions such as emphysema, bronchitis, and asthma . It can inflame breathing passages, decrease the lungs' working capacity, cause shortness of breath, pain when inhaling deeply, wheezing, and coughing . It can cause eye and nose irritation and it dries out the protective membranes of the nose and throat and interferes with the body's ability to fight infection, increasing susceptibility to illness . Hospital admissions and respiratory deaths often increase during periods when ozone levels are high . There is a lack of knowledge on the long - term effects of air pollution exposure and the origin of asthma . An experiment was carried out using intense air pollution similar to that of the 1952 Great Smog of London . The results from this experiment concluded that there is a link between early - life pollution exposure that leads to the development of asthma, Proposing the ongoing effect of the Great Smog . Modern studies continue to find links between mortality and the presence of smog . One study, published in Nature magazine, found that smog episodes in the city of Jinan, a large city in eastern China, during 2011--15, were associated with a 5.87% (95% CI 0.16--11.58%) increase in the rate of overall mortality . This study highlights the effect of exposure to air pollution on the rate of mortality in China . </P> <P> The U.S. EPA has developed an Air Quality Index to help explain air pollution levels to the general public . 8 hour average ozone concentrations of 85 to 104 ppbv are described as "Unhealthy for Sensitive Groups", 105 ppbv to 124 ppbv as "unhealthy" and 125 ppb to 404 ppb as "very unhealthy". The "very unhealthy" range for some other pollutants are: 355 μg m--424 μg m for PM10; 15.5 ppm--30.4 ppm for CO and 0.65 ppm--1.24 ppm for NO . </P> <P> In 2016, the Ontario Medical Association announced that smog is responsible for an estimated 9,500 premature deaths in the province each year . </P> <P> A 20 - year American Cancer Society study found that cumulative exposure also increases the likelihood of premature death from a respiratory disease, implying the 8 - hour standard may be insufficient . </P>

A secondary reaction in the formation of photochemical smog is