What can possibly kill sperm in the great sperm race?

THE GREAT SPERM RACE:

A sperm's race to fertilize an egg is not so easy. Out of about 250 million sperm ejaculated into the human vagina during intercourse, not more than one in a hundred will survive the Great race to the end due to the hurdles it has to face in the hostile, ACIDIC CHAMBER to the cervix (it has hundreds of tiny branching tunnels that can trap, crush and slowly kill sperm).

If ovulation is not occurring soon the sperm will "drown in a thick flow of cervical mucus

significant amounts of nitrogen oxides, carbon monoxide, and other pollution

explanation:

passenger vehicles are a major pollution contributor, producing significant amounts of nitrogen oxides, carbon monoxide, and other pollution. in 2013, transportation contributed more than half of the carbon monoxide and nitrogen oxides, and almost a quarter of the hydrocarbons emitted into our air.