On the 22nd of September 2021 the WHO published their updated review of air quality guidelines. Following a detailed systematic review of the accumulated evidence since their last Global update in 2005 they significantly reduced both the short (24 hour) and long-term (annual) recommendations for PM2.5, from 25 to 15 and 10 to 5 mg/m3 respectively. Given that over 99% of the global population already lived in areas failing to meet the previous annual guideline concentration for PM2.5 (10 mg/m3), here I will review the consequences of this halving of the annual guideline value and the evidence that drove this decision. I will also explore the impacts of this decision on the current UK Government Clean Air strategy that aimed to half the number of people living in locations with annual exposures above the former PM2.5 guideline value by 2025 and the challenges of moving toward this new target.

1. Revisiting PM2.5 guidelines, the evidence,
and the consequences
Ian Mudway – Imperial College London

3. 10 µg/m3

6. “The government is committed to evidence-based policy making, and will consider
the WHO’s annual mean guideline level for PM2.5 when setting the target, alongside
independent expert advice, evidence and analysis on a diversity of factors – from the
health benefits of reducing PM2.5, to the practical feasibility and economic viability
of taking different actions.”
10 µg/m3
25 µg/m3

7. Proc Natl Acad Sci U S A. 2018 Sep 18;115(38):9592-9597. Proc Natl Acad Sci U S A. 2018 Sep 18;115(38):9592-9597
Association between long-term PM2.5 exposure and
mortality from NCDs and lower respiratory illness. Data
from 41 different cohort studies
Evidence of health effects below the former WHO guideline
value for PM2.5
Relationship between long-term PM2.5 exposures and all
cause mortality in the USA Medicare pop: 60 million +
Di et al. N Engl J Med. 2017;376(26):2513-2522. Burnett et al. Proc Natl Acad Sci U S A. 2018;115(38):9592-9597.

8. Recommended AQG – with interim targets

9. Proc Natl Acad Sci U S A. 2018 Sep 18;115(38):9592-9597. Proc Natl Acad Sci U S A. 2018 Sep 18;115(38):9592-9597
PM
2.5
impacts
Jie Chen, Gerard Hoek, Env Int. 143 (2020) 105974

10. Proc Natl Acad Sci U S A. 2018 Sep 18;115(38):9592-9597. Proc Natl Acad Sci U S A. 2018 Sep 18;115(38):9592-9597
PM
2.5
impacts
Jie Chen, Gerard Hoek, Env Int. 143 (2020) 105974
1.08 (1.06, 1.09) per 10 μg/m3 increase
in PM2.5

11. Proc Natl Acad Sci U S A. 2018 Sep 18;115(38):9592-9597. Proc Natl Acad Sci U S A. 2018 Sep 18;115(38):9592-9597
PM
2.5
impacts
Jie Chen, Gerard Hoek, Env Int. 143 (2020) 105974

12. Proc Natl Acad Sci U S A. 2018 Sep 18;115(38):9592-9597. Proc Natl Acad Sci U S A. 2018 Sep 18;115(38):9592-9597
PM
2.5
impacts
Jie Chen, Gerard Hoek, Env Int. 143 (2020) 105974
1.11 (1.09, 1.14) per 10 μg/m3 increase
in PM2.5

13. Proc Natl Acad Sci U S A. 2018 Sep 18;115(38):9592-9597. Proc Natl Acad Sci U S A. 2018 Sep 18;115(38):9592-9597
PM
2.5
impacts
Jie Chen, Gerard Hoek, Env Int. 143 (2020) 105974

14. Proc Natl Acad Sci U S A. 2018 Sep 18;115(38):9592-9597. Proc Natl Acad Sci U S A. 2018 Sep 18;115(38):9592-9597
PM
2.5
impacts
Jie Chen, Gerard Hoek, Env Int. 143 (2020) 105974
1.10 (1.03, 1.18) per 10 μg/m3 increase
in PM2.5

17. Reflections
• Particle number concentration, BC/EC and NO2
• Feasibility versus credibility gap
• Sources and components – versus an absolute focus on mass
• Interim targets
• Indoor versus outdoor air
“However, as in previous editions, these guidelines do not cover occupational
settings, owing to the specific characteristics of the relevant exposures and
risk reduction policies and to potential differences in population susceptibility
of the adult workforce in comparison with the general population.”