Everyone is exposed to air pollution but some groups of people may be more susceptible to harmful effects of air pollutants than others. How do we know whether one individual is more susceptible to harmful effects of air pollution than another? To answer this question, epidemiological studies conduct stratified analysis. A Stronger association between air pollution and health specific health outcome in one subgroup compared with another, provides evidence for a more susceptible population. Following characteristics of populations have been associated with increased susceptibility to air pollution health effects. In this module, I will discuss each of these characteristics and show an example of how susceptibility is determined in an epidemiological study. Children exposed to comparable levels of air pollution are more susceptible to its harmful effects than are adults because they spend more time outdoors, do more physical activity, and have greater respiratory rates. All this can lead to an increased air pollution dose per lung surface area. Moreover, children's lungs and immune system are not fully developed, making them more susceptible to air pollution affects. 80 percent of alveoli develop between birth and age of six years. Immune system continues to develop during early childhood, which is apparent in higher rate of infections. For these reasons, infants and toddlers are also the most susceptible, and air pollution can even lead to respiratory deaths. Compared to adults, children exposed to air pollution experience higher rates of chronic cough and bronchitis, wheezing, and asthma attacks, and respiratory related school absences, as well as reduced lung function. In a study in 2005, children in Copenhagen were linked to daily concentrations of nitrogen dioxide, to daily diary data on wheezing from birth until age three. For each six parts per billion increase in NO2, we found 33 percent higher odds of having wheezing symptoms in infants below age of one, and no increased risk in wheezing due to NO2 children aged two and three years, showing that the youngest of children are most susceptible to traffic related air pollution. Elderly adults, typically defined as people above age 65, are consider a susceptible population, because of gradual decline in multiple physiological processes with age. Aging is a continuous process of progressive decline of the body's functions, including accelerated decline in lung function, weakening of the immune system and its ability to respond to infections, all of which, make elderly more susceptible to harmful effects of air pollution. Elderly subjects also represent a susceptible population compared to younger adults, because of the higher rates of pre-existing chronic cardiovascular and respiratory diseases. In an example, authors reviewed all studies of association between short term exposure to ozone, and mortality and performed separate analysis by age, separating results for younger and older subjects. They found that association between ozone and mortality, were statistically different by age. For each 10 parts per billion increase in eight hour ozone concentration, risk of dying the same day was increased by 0.6 percent in younger persons, and by 1.3 percent in older persons. Elderly people exposed to short term increases in air pollution experienced higher rates of dying and hospitalizations for respiratory and cardiovascular disease, and pneumonia than younger adults. With people living longer, older people are making up an increasing share of the total population. More and more people will live longer with one or more chronic diseases. Therefore, the air pollution burden and this susceptible group will be a major public health concern and is a main driver of global increase in health burden due to air pollution. People with pre-existing conditions are not a are susceptible population. A study from Chicago USA, examine association between short term exposure to V.M. 10 and hospital admissions for heart disease. Between 1988 and 1994 separately for persons with and without diabetes. The researchers found that a 10 micrograms per cubic meter increase in PM 10, was associated with a two percent increase in admissions for heart disease in persons with diabetes, but only 0.9 Increase in persons without diabetes and this difference was so statistically significant, and seen in both subjects below and above age 75. On the basis of this finding authors concluded that persons with diabetes are susceptible population. Similarly, studies have shown that asthmatics of all ages are considerably more likely to experience wheezing and need for medication or hospital care, following exposure to air pollution, than people without asthma. People with pre-existing chronic obstructive respiratory disease and cardiovascular disease are more likely to experience worsening of their disease, leading to medication use, hospitalizations, or death as a result of exposure to air pollution than healthy subjects. Pregnancy increases susceptibility to air pollution and both mother and unborn child are susceptible. Exposure to high levels of air pollution during pregnancy can lead to lower birth weight and pre-term birth. Children born before term have a significant risk of neurological disorders and permanent physical disabilities. Some evidence exists that fetal air pollution exposure increases risk of autism, asthma, and learning difficulties. Moreover, exposure to high levels of air pollution during pregnancy can increase risk of pregnancy complications, and increase women's risk of gestational diabetes, hypertension, and pre-eclampsia. All dangerous conditions for both mother and baby. Socioeconomic status measured by income, education, and occupation can influence one's susceptibility to air pollution. Low socioeconomic status is associated with poor lifestyle. For example; high smoking and drinking rates, obesity, low physical activity, and limited access to healthy food, and resulting in higher rates of pre-existing disease. People with low socioeconomic status have poor access to medical care and often reside in poor neighborhoods with high levels of air pollution. Studies have shown that people with low socioeconomic status, experience higher risk of mortality and mobility due to air pollution, than people with high socioeconomic status. There are also other factors being started for susceptibility to air pollution. For example, there is an increasing interest for understanding to potential gene environment interactions in the relationship between air pollution and health, and identifying genetic polymorphisms which are more susceptible to air pollution. Other factors that have been started with respect to susceptibility to air pollution include; obesity, gender, diet, smoking, and ethnicity. But evidence precludes conclusions. Thank you for watching.