According to the Central Pollution Control Board CPCB, particulate matter of what size in micrometers or less in diameter causes the greatest harm to human health?

Introduction / Context:
Air pollution has become a major environmental and health concern in modern cities. Exams frequently ask about particulate matter and standards used by agencies like the Central Pollution Control Board CPCB and the World Health Organization. This question focuses on the size category of particles that cause maximum health damage, particularly by entering deep into the lungs and blood stream.

Given Data / Assumptions:

• The question talks about particulate size measured in micrometers microns.
• We are asked which size or smaller is most harmful to human health.
• The options range from 0.5 micrometers to 10 micrometers.
• We assume CPCB classification of particulate matter such as PM10 and PM2.5.

Concept / Approach:
Particulate matter is usually grouped in categories such as PM10 particles of size 10 micrometers or less and PM2.5 particles of size 2.5 micrometers or less. The smaller the particle, the more deeply it can penetrate into the respiratory system. PM2.5 particles can reach the alveoli of the lungs and may even enter the blood stream, causing cardiovascular and respiratory diseases. CPCB and WHO guidelines therefore emphasise PM2.5 as the most critical health concern.

Step-by-Step Solution:
Step 1: Recall that PM10 refers to particulate matter of size up to 10 micrometers. Step 2: Recall that PM2.5 refers to finer particulate matter of size up to 2.5 micrometers. Step 3: Understand that finer particles like PM2.5 can travel deeper into the lungs compared to coarser PM10 particles. Step 4: Recognise that CPCB standards and air quality indices highlight PM2.5 as a major indicator of harmful air pollution. Step 5: Based on this, choose 2.5 micrometers or less as causing the greatest harm to human health.

Verification / Alternative check:
Any official document or environmental science text that explains PM10 and PM2.5 will mention that PM2.5 is more dangerous because of its ability to penetrate deeper into the respiratory tract. Air quality index bulletins often report PM2.5 levels prominently. This repeated emphasis on PM2.5 confirms that 2.5 micrometers or less is considered most harmful for health among the commonly measured particulate sizes.

Why Other Options Are Wrong:
0.5 micrometers or less: Particles can be this small, but CPCB and common standards focus on PM2.5 and PM10 rather than a PM0.5 category, so this is not the usual regulatory emphasis.
5 micrometers or less: This size lies between PM2.5 and PM10 but is not the standard regulatory cutoff for most harmful particulates.
10 micrometers or less: These particles PM10 are harmful but generally considered less damaging than PM2.5, because many are filtered by the upper respiratory tract.

Common Pitfalls:
Candidates sometimes pick 10 micrometers or less because they remember hearing the term PM10. However, for health impact, PM2.5 is more critical. Another mistake is not paying attention to the number attached to the PM term. A clear strategy is to remember that the smaller the particle size number, the deeper the penetration into the lungs and the greater the health risk.

Final Answer:
The most harmful particles for human health as per CPCB standards are those of size 2.5 micrometers or less, that is PM2.5.