At a Glance
Four facts to hold on to.
PM2.5 (WHO 2021)
5 µg/m³ annual; 15 µg/m³ 24-hour.
India NAAQS PM2.5
40 µg/m³ annual; 60 µg/m³ 24-hour.
Ozone
Forms in sunlight; can trigger asthma.
UPSC 2022
Statements 1 and 4 only.
In this lesson
What the Numbers Mean
A guideline is not a law
- An air-quality guideline is a health-based level for pollution in air.
- WHO published a major update in 2021.
- It is not a promise of zero risk. Risk can rise even below it.
- It does not automatically apply in India as law.
- India has its own NAAQS, the National Ambient Air Quality Standards.
- A monitor measures outdoor concentration. It does not measure what entered one person’s blood.
- Sources: WHO Q&A and CPCB NAAQS.
Read each value with its averaging time
| Pollutant | WHO 2021 guideline | Meaning |
|---|---|---|
| PM2.5 | 5 µg/m³ annual; 15 µg/m³ 24-hour | Fine particles over a year or a day |
| PM10 | 15 µg/m³ annual; 45 µg/m³ 24-hour | Particles up to 10 micrometres |
| Ground-level ozone | 100 µg/m³ 8-hour; 60 µg/m³ peak season | Short period and high-ozone season use different averages |
- WHO’s short-period values use a 99th-percentile rule.
- That allows about three or four exceedance days a year.
True or False: WHO’s 2021 guidelines set the annual PM2.5 mean at no more than 5 µg/m³.
Particles, Ozone and Health
PM10 and PM2.5
- Particulate matter is a mix of tiny solid and liquid bits in air.
- PM10 includes particles up to 10 micrometres. PM2.5 is part of PM10, not a separate pollution.
- Large dust tends to settle in upper airways.
- Fine particles can reach deep into the lungs.
- Some ultrafine particles, and dissolved chemicals, may enter blood.
- Most health effects do not need whole particles in blood. Lung inflammation can send signals through the body.
- Sources: US EPA, particle sizes, US EPA, exposure and WHO health pathways.
Ground-level ozone
- Ozone high in the sky blocks harmful ultraviolet rays.
- Ground-level ozone forms when gases from traffic and industry react in sunlight.
- Hot, sunny days favour its build-up.
- Too much ozone can irritate airways and trigger asthma.
- Source: WHO pollutant guide.
Why WHO revised the guide
- The 2021 update followed the 2005 guidelines.
- New studies showed harm at lower levels.
- An interim target is a nearer goal. It does not say higher pollution is safe.
True or False: WHO’s 2021 revision was its first update of these guidelines since 2005.
Previous Year Questions
UPSC 2022 asked about WHO air-quality guidelines.
- UPSC 2022Statement 1 (PM2.5: 24-hour 15, annual 5 µg/m³) is correct. Statement 2 (ozone peaks in inclement weather) is wrong, because sunlight helps ozone form. Statement 3 (PM10 penetrates the lung barrier into blood) is marked wrong by the exam. Statement 4 (excess ozone can trigger asthma) is correct. The answer is statements 1 and 4 only. PM10 includes PM2.5, and some ultrafine particles can enter blood, so do not treat the exam wording as absolute. View the question.
True or False: UPSC 2022 confirmed that ozone pollution peaks during inclement, rainy weather.
True or False: Excessive ozone can trigger asthma attacks.
India and the WHO Guide
- India’s NAAQS annual PM2.5 limit is 40 µg/m³. WHO’s is 5 µg/m³. Forty is eight times five.
- India’s 24-hour value is 60 µg/m³. WHO’s is 15 µg/m³.
- This compares two standards, not the air in one town.
- A national standard is a legal benchmark. WHO guidance is a health reference.
- Sources: CPCB and WHO.
A dated ranking
- IQAir’s 2024 report ranked Byrnihat first among measured metropolitan areas.
- Its annual PM2.5 average was 128.2 µg/m³, about 25.6 times WHO’s guide.
- This is one company’s report, not an all-time verdict.
- Rankings depend on monitor coverage.
- Source: IQAir 2024 World Air Quality Report.
Think it through: why can PM2.5 harm the heart even if most particles stay in the lungs?
Fine particles inflame deep lung tissue. The body then sends chemical signals into the blood. Some ultrafine particles or dissolved chemicals may also enter blood. Direct entry is one pathway, not the only one.
True or False: India’s NAAQS allow an annual PM2.5 mean of 40 µg/m³, eight times WHO’s guideline.
True or False: IQAir ranked Byrnihat highest among measured metropolitan areas in its 2024 report.
True or False: PM10 includes PM2.5, while ultrafine particles are the most able to move from lungs into blood.
Quick Q&A
WHO annual PM2.5?
5 µg/m³.
India annual PM2.5?
40 µg/m³.
Does ozone peak in rain?
No, in sunlight.
PM2.5 inside PM10?
Yes.
UPSC 2022 answer?
1 and 4 only.
🎲 Practice: WHO Air Quality Guidelines: PM2.5, Ozone, and What Actually Enters the Bloodstream
3 verified questions from this topic.
Question 1 of 1
Question 1 of 1
Question 1 of 1
Leave a Reply