As per the National Ambient Air Quality Standards (NAAQS) 2009, the 24-hour average limit for sulphur dioxide in industrial, residential and rural areas of India is:

• A 50 micrograms per cubic metre
• B 80 micrograms per cubic metre ✓
• C 100 micrograms per cubic metre
• D 120 micrograms per cubic metre

Explanation

Under NAAQS 2009, the 24-hour standard for sulphur dioxide is 80 micrograms per cubic metre and the annual standard is 50 micrograms per cubic metre for industrial, residential, rural and other areas. For ecologically sensitive areas the annual limit is tightened to 20 micrograms per cubic metre while the 24-hour value remains 80. The distractor of 50 micrograms per cubic metre is the annual mean, not the 24-hour mean, which is what the stem asks.

Reference: Park's Textbook of Preventive and Social Medicine, 26th ed.

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