Breathing difficulty – The presence of CO in the air can deprive the hemoglobin of the human body of the needed oxygen and so results in breathing difficulty.
Greenhouse effect – The presence of some gases which are denser than air like CO can cause the retaining of heat radiation like ultraviolet rays in the atmosphere to cause much heating of the environment even when the sunsets.
Headache – It can cause headaches in human beings.
Acid rains – Where gases like SO2, NO3 etc are present in the atmosphere, when it rains they will always dissolve there to form acidic rains since they are acid anhydride.