One of the main causes of air pollution in cities is emissions from ve

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 (NO_x)
2. Carbon monoxide (CO)
3. Carbon dioxide (CO₂)
4. Benzene

Which of the above pollutants are not tolerated by human beings even at very low levels ?

1 and 4 only

1 and 2 only

1, 2 and 3 only

1, 2 and 4 only

This question was previously asked in

UPSC NDA-1 – 2016

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The question asks which pollutants are *not* tolerated by human beings *even at very low levels*. This typically refers to substances with high toxicity or carcinogenicity at trace concentrations.
1. Nitrogen oxides (NOx): Respiratory irritants, contribute to smog. Can cause respiratory problems at relatively low concentrations.
2. Carbon monoxide (CO): Highly toxic gas. Binds to haemoglobin more strongly than oxygen, impairing oxygen transport. Even low levels in inhaled air can be dangerous, especially over time.
3. Carbon dioxide (CO₂): A greenhouse gas and a natural component of air (around 400 ppm). It is not toxic at these ambient levels. High concentrations (e.g., > 5000 ppm or more) can cause adverse health effects or suffocation, but it is tolerated at “very low levels” (ambient levels).
4. Benzene: A volatile organic compound and a known human carcinogen. Exposure to benzene, even at very low concentrations over time, is linked to increased risk of cancer (e.g., leukemia).
Considering the criteria “not tolerated even at very low levels”, CO and Benzene are clear candidates due to their high toxicity/carcinogenicity at low exposures. NOx are also significant respiratory hazards at lower concentrations than CO₂. CO₂ is relatively non-toxic at typical atmospheric concentrations. Thus, NOx, CO, and Benzene fit the description better than CO₂.

Certain pollutants like carbon monoxide (CO), benzene, and nitrogen oxides (NOx) pose health risks even at low exposure levels due to their toxicity, carcinogenicity, or irritant properties, whereas carbon dioxide (CO₂) is harmful primarily at high concentrations.

The term “tolerated at very low levels” is relative, but in the context of common air pollutants from vehicles, CO, NOx, and Benzene are regulated and considered hazardous at much lower concentrations than CO₂. CO₂ is regulated more as a climate change gas than a direct air toxic at ambient urban levels.

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