Safe thresholds of particular pollutants have been put into place by the World Health Organization (WHO). These international standards are designed to provide appropriate targets for country policymakers to reduce the health impacts of air pollution on their populations.  As such, these can be seen as benchmarks for what is viewed as healthy levels of each pollutant and therefore goals for each country to strive for. These benchmarks can be directly compared to the measurements of individual pollutants reported from various sites around Delhi. For example, WHO data showed Delhi has an annual average concentration PM 2.5 at 153 micrograms per cubic meter and the WHO international benchmarks for PM 2.5 is 10 micrograms per cubic meter.  Particulate matter, ozone, nitrogen dioxide and sulfur dioxide are discussed in depth in the full report here.

To see WHO benchmarks and Standards, scroll down.

Countries also develop their own standards. India’s National Ambient Air Quality Standards (NAAQs) can be seen in the chart below created by Center for Science and Environment. This chart compares NAAQs to the standards of the WHO and US EPA. Notice India’s relatively strict standards for carbon monoxide, but relatively lax standards for particulate matter.

CSE 2013

Source: Good News, Bad News, Clearing the Air in Indian Cities, The Center for Science and Environment, 2015

WHO International Benchmarks
Particulate Matter
PM 2.5
10 μg/m3 annual mean
25 μg/m3 24-hour mean

20 μg/m3 annual mean
50 μg/m3 24-hour mean
Rationale: So far there is no evidence to suggest a threshold below which no adverse health effects can be anticipated. In other words, particulate matter of any concentration is bad for health.  The guidelines for particulate matter therefore, aim to achieve the lowest concentrations of particulate matter possible.

O3: 100 μg/m3 8-hour mean
Rationale: With new evidence to weigh, this guideline has recently been lowered from 150 μg/m3 8-hour mean to 100 μg/m3 8-hour mean.  As ozone concentrations increase above the new guideline value, health effects at the population level become increasingly numerous and severe.

Nitrogen dioxide
40 μg/m3 annual mean
200 μg/m3 1-hour mean
Rationale: Studies show that at short-term concentrations higher than benchmark levels, nitrogen dioxide is a toxic gas that can cause adverse effects to health. There is still no robust basis for setting an annual average guideline for NO2 through any direct toxic effect. Evidence has emerged, however, that increases the concern over health effects associated with outdoor air pollution mixtures that include NO2. For instance, epidemiological studies have shown that bronchitic symptoms of asthmatic children increase in association with annual NO2 concentration and that reduced lung function growth in children in linked to elevated NO2 concentrations.

Sulfur Dioxide  (SO2)
20 μg/m3  24-hour mean
500 μg/m3   10-minute mean
Rationale: Studies involving exercising asthmatics indicate that a proportion experience changes in pulmonary function and respiratory symptoms after periods of exposure to SO2 as short as ten minutes. With regards to the long term benchmark, the latest evidence to emerge includes a study conducted in Hong Kong where a major reduction in the sulfur content of fuels has been achieved over a very short period of time. This has been linked to substantial reductions in health effects (e.g. childhood respiratory disease and all-age mortality). There is still considerable uncertainty as to whether SO2 is the pollutant responsible for the observed adverse effects or whether it is a surrogate for ultrafine particles or some other correlated substance.

How do I make sense of the numbers in Delhi?
As you can see from the above, air pollution measurements in Delhi far exceed the WHO international benchmarks and standards on almost any day. To make some sense of how to interpret the numbers, visit the page on Activity Guidelines.