Q: 10 (NDA-I/2016)

One of the main causes of air pollution in cities is emissions from vehicles like cars and trucks. Cars emit various pollutants which are bad for human health when inhaled, like
1. Nitrogen oxides (NOx)
2. Carbon monoxide (CO)
3. Carbon dioxide (C02) .
4. Benzene
Which of the above pollutants are not tolerated by human beings even at very low levels ?
question_subject: 
Science
stats: 
0,512,1488,136,216,888,760