Which gaseous pollutant,even in low concentrations,causes respiratory distress such as inflammation and irritation in the human respiratory tract?

Smog is generally produced in the presence of which of the following?

Some statements are given:
$(S_1) \ CO_2$ is a greenhouse gas.
$(S_2)$ Classical smog contains smoke,fog,and a high concentration of $SO_2$.
$(S_3) \ DDT$ is biodegradable.
$(S_4)$ Drinking water should have a high $B.O.D.$
Which is the correct sequence of true $(T)$ or false $(F)$ regarding the above statements?

What can we do to reduce the rate of global warming?

$A$ water sample is reported to be highly polluted if the $BOD$ (Biological Oxygen Demand) value of the sample becomes:

Explain the formation of photochemical smog with reactions.