Heterogeneities are possibly due to systematic errors in exposure ascertainment, unaccounted for differences in the study populations (genetic, lifestyle, etc.), or confounding mine exposures.  There are a number of confounding factors to consider, including exposure to other agents, ethnicity, smoking history, and work experience. The cases reported in these miners cannot be attributed solely to radon or radon daughters but may be due to exposure to silica, to other mine pollutants, to smoking, or to other causes.   The majority of miners in the studies are smokers and all inhale dust and other pollutants in mines. Because radon and cigarette smoke both cause lung-cancer, and since the effect of smoking is far above that of radon, it is complicated to disentangle the effects of the two kinds of exposure; misinterpreting the smoking habit by a few percent can blur out the radon effect. 
They occur mainly in the form of sulphur dioxide. It is produced in large quantity during smelting of metallic ores and burning of petroleum and coal in industries, thermal plants, home and motor vehicles. In the air, SO 2 combines with water to form sulphurous acid (H 2 SO 3 ) which is the cause of acid rain. It causes chlorosis and necrosis of vegetation. Sulphur dioxide, above 1 ppm, affects human beings. It causes irritation to eyes and injury to respiratory tract. It results in discolouration and deterioration of buildings, sculptures, painted surfaces, fabrics, paper, leather, etc.
Animal and Vegetation:
Animal digestion (particularly by cattle) is another cause of natural air pollution, leading to the release of methane, another greenhouse gas. In some regions of the world, vegetation – such as black gum, poplar, oak, and willow trees – emits significant amounts of volatile organic compounds (VOCs) on warmer days. These react with primary anthropogenic pollutants – specifically nitrogen oxides, sulfur dioxide and carbon compounds – to produce low-lying seasonal hazes that are rich in ozone.