One of the effects of an increasing temperature trend in cities is the increasing level of ozone in the air, assuming that motor vehicle emissions remain at current levels. Ozone is a major pollutant formed when emissions from motor vehicles and UV radiation from sunshine react in air (WHO, 2005c). Its formation is faster and greater when the air temperature increases. Ozone is one of the major air pollutants that increase the incidence and mortality of heart and lung diseases as well as causing respiratory irritation symptoms (WHO, 2006). The efforts to reduce health-damaging air pollutants as well as greenhouse gases in motor vehicle exhaust will most likely reduce emissions from new vehicles, but in many cities older vehicles will remain on the roads for a long time. In addition, the increasing number of vehicles in many cities, particularly in developing countries, will counteract the improvements in engine technology and total pollution emissions in an area will still increase. Thus, it is prudent to assume that vehicle emissions in major cities will not decrease and that climate change will lead to increasing ozone exposures for residents. Air pollution effects need to be analysed as well because they are likely to interact or confound the weather effects on mortality.