When the reduction of intensity is accomplished by removal of odorant from the atmosphere, the process is equivalent to gas and vapor abatement, or to air cleaning, but with some special considerations. These include(1) problems related to the need to attain very low concentrations, often approaching threshold levels, (2) uncertainties with regard to the reliability of sensory or chemical analyses(see Section 4), and difficulties sociated with diffuse or sporadic sources. When an odorous atmosphere is improved by the addition of another(usually pleasant) odorant under conditions in which chemical reactions are notinvolved, the process is called odor modification, referring to modification of the odor perception not of the odorant. These and other methods of or control are considered in some detail in Section 6