Which are possible consequences of decreasing air quality resulting from global

increase in illnesses

decrease in population due to health threats

so 1 and 5

    Increase in illnesses

                  Decrease in population due to health threats

Explanation: The air quality is responsible for describing about the pollutant present in the air and extent to which the air is free from it .It is not highly risky situation to have low pollution in air but as the degradation of air quality takes place in continuation, risk also increases.

The consequences that can occur due to this situation are the diseases like asthma ,cancer etc because of long exposure to pollution and hazardous environment.These illness states can also lead to death and thus population decrement can happen.

Other options are incorrect because fertile land will not dry, flooding and glacier melting does not take place due to air quality decrement.