Whenever the humidity is high air movement is less and the temperature is low fog is the automatic choice. It reduces visibility and more accidents are reported on the roads and delay in both railway and airlines services. When the level of pollution is high in the atmosphere the pollutant particles get mixed into the fog reducing its visibility further and the resultant is called as smog. Smog is very dangerous to both lungs and the heart. High sulfur dioxide contents lead to precipitation of chronic bronchitis. High nitrogen dioxide contents lead to precipitation of asthma. High particulate matter PM10 air pollutants sized between 2.5 to 10 microns can damage the lungs and precipitate asthma due to air flow and inflammation. High contents of particulate matter PM 2.5 air pollutants less than 2.5 micron in size can enter the lungs and damage the lung lining. They can also be absorbed and cause inflammation of the heart arteries. Patients with asthma and chronic bronchitis should get the dose of their medicine increased during smog days. Avoid exertion in conditions of smog. It is better to avoid walking during smog hours. Drive slowly during smog hours. Heart patients should stop their early morning walk during smog hours. Remember to take the flu pneumonia vaccine.