PM2.5, the pollutant reported in air quality bulletins, refers to particles whose diameter is:
- A.2.5 millimetres or less
- B.2.5 nanometres or less
- C.2.5 micrometres or less
- D.2.5 centimetres or less
Correct answer
Explanation
The correct answer is C, 2.5 micrometres or less. Particulate matter is graded by the diameter of the particles measured in micrometres, so PM10 means particles of ten micrometres or less and PM2.5 means particles of 2.5 micrometres or less, which is about thirty times finer than a human hair. This fine fraction matters most for health because the nose and throat cannot filter it out; it travels deep into the lungs, settles in the alveoli and can pass into the bloodstream. Option A is wrong because a millimetre is a thousand times larger than a micrometre, a size no longer suspended in air. Option B is wrong because a nanometre is a thousand times smaller. Option D is wrong because a centimetre-sized particle is not particulate matter at all.
Read the full article: Air Pollution and Air Quality: Pollutants, AQI and Laws
Practice Questions
View allWhich of the following is a secondary air pollutant?
- A.Sulphur dioxide
- B.Carbon monoxide
- C.Peroxyacetyl nitrate
- D.Particulate matter
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A secondary pollutant is never emitted from a source; it is manufactured in the atmosphere itself when primary pollutants react with one another or with sunlight.
Read the full explanation →Which of the following is the chief component of photochemical smog?
- A.Sulphur dioxide
- B.Ozone
- C.Methane
- D.Carbon dioxide
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Photochemical smog, also called Los Angeles smog, forms in warm, dry, sunny weather when oxides of nitrogen from vehicle exhaust and unburnt hydrocarbons react under ultraviolet light.
Read the full explanation →Carbon monoxide is dangerous to human beings mainly because it:
- A.Combines with haemoglobin to form carboxyhaemoglobin
- B.Dissolves in water to form a strong acid
- C.Destroys the ozone layer of the stratosphere
- D.Settles in the alveoli as fine dust
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Carbon monoxide binds to the haemoglobin of the blood roughly two hundred times more readily than oxygen does, and the carboxyhaemoglobin so formed cannot carry oxygen.
Read the full explanation →Along with the oxides of nitrogen, which gas is chiefly responsible for acid rain?
- A.Carbon monoxide
- B.Methane
- C.Sulphur dioxide
- D.Argon
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Sulphur dioxide, released when coal and oil containing sulphur are burnt, is oxidised in the atmosphere to sulphur trioxide, which dissolves in water droplets to give sulphuric acid.
Read the full explanation →The damage to the marble of the Taj Mahal, popularly called 'marble cancer', is caused chiefly by:
- A.Carbon dioxide
- B.Sulphur dioxide
- C.Chlorofluorocarbons
- D.Ground-level ozone
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Sulphur dioxide from industry and traffic around Agra is oxidised in the air and comes down as sulphuric acid, which attacks the calcium carbonate of marble.
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