The Union Carbide gas leak in Bhopal was a tragedy of gargantuan proportions and the loss of life and damage to survivors resulting from it cannot be easily forgotten. The nerve gas attack on the Japanese metro was another shocking incident. We know that human activity involves the handling of many hazardous chemicals. There is always the possibility that gross human error or diabolism can unleash a disaster. Education and awareness can help avoid these.
Long term effects of the slow but surely increasing toxicity is pretty serious too. We are all contributing to this, and suffering too, in an inevitable cycle. Air pollution, not surprisingly, affects the respiratory system. Diseases like chronic bronchitis and primary lung cancer are suspected to be related to long term inhalation of polluted air.
Plants and Animals: Unfortunately, mankind's industrial progress has not been very beneficial to the other lives that share the planet with him. Plants are very sensitive to sulphur dioxide, fluorine compounds, smog etc. Spotting and burning of leaves, destruction of crops, and retarded growth of plants have been observed. Flourides are very toxic to animals. Cattle suffer by eating foliage contaminated with fluorides.
Fringe problems: Building grills corrode. Buildings get grimy. Your white clothes don't remain white if you travel in open vehicles in any Indian metro. There is always a bad odour hanging in the air.