Air quality is a serious problems in major cities

There’s no such thing as fresh air in Delhi, the Indian capital right now, with toxic smog reaching “unbearable levels” at the weekend. 

The concentration of harmful PM 2.5 particles in the atmosphere hit 810 micrograms per cubic metre (mpcm) on Sunday morning. To provide a little context, the highest reading anywhere in London is currently 65, the recommended World Health Organisation safe daily maximum is 25, and anything above 300 is considered “hazardous”. 

Every winter, the megacity of 20 million people is blanketed by car fumes, industrial emissions and smoke from stubble burning at farms in neighbouring states. PM 2.5 refers to fine particles (2.5 micrometres or smaller in diameter), which are produced by combustion, including motor vehicles, power plants, forest fires, and some industrial processes.

Air quality or ambient/outdoor air pollution is represented by the annual mean concentration of particulate matter PM10 (particles smaller than 10 microns) and PM2.5 (particles smaller than 2.5 microns, about 25 to 100 times thinner than a human hair).

The world's average PM10 levels, for the period 2008 and 2013, based on data of 1600 cities in 91 countries, range from 26 to 208 micrograms per cubic meter of air (μg/m3), with the world average being 71 μg/m3. 13 of the 25 cities worldwide with the highest levels of PM are in India.

In 2010, the year of the WHO survey, the average PM10 level in Delhi was 286 μg/m3. In 2013, the PM2.5 level was 153 μg/m3. These levels are considered very unhealthy. In Gwalior, the city with the worst air quality in India, the PM10 and PM2.5 levels were 329 μg/m3 and 144 μg/m3 respectively. For comparison, the PM10 and PM2.5 levels in London were 22 μg/m3 and 16 μg/m3 respectively. The PM levels in Delhi have become worse since the WHO survey. In December–January 2015, in Delhi, an average PM2.5 level of 226 μg/m3 was noted by US embassy monitors in Delhi. The average in Beijing for the same period was 95. Delhi's air is twice as bad as Beijing's air. As of October 2017, experts in several monitoring stations have reportedly measured an air quality index (AQI) of 999. According to said experts this is the equivalent of smoking 45 to 50 cigarettes a day. This has led to some government officials, such as Arvind Kejriwal calling the nation's capital a "gas chamber".

Safe levels for PM according to the WHO's air quality guidelines are 20 μg/m3(annual mean) for PM10 and 10 μg/m3 (annual mean) for PM2.5.

Measuring Air Quality: the Air Quality Index

One of the main indexes utilized to measure air quality is the US Air Quality Index (developed by the Environmental Protection Agency of the United States) which takes into consideration the concentration and duration of 6 parameters: 

O3 (ozone), PM2.5, PM10, CO, SO2 and NO2

According to this index, 6 levels of pollution are defined as follows.

0-50    Excellent air quality

51-100 Good air quality

100-150 Lightly polluted

151-200 Moderately polluted

201-300 Heavily polluted

More than 300 Severely polluted

(see table below)

Heavily polluted cities of the world

According to this index and data obtained from different major cities worldwide the 10 most heavily polluted cities are:

Hanoi (Vietnam): AQI – 199

Ulaanbaatar (Mongolia): AQI- 191

Lahore (Pakistan): AQI – 190

Jakarta (Indonesia): AQI – 185

Krasnoyarsk (Rusia): AQI – 179

Kabul (Afganistán):  AQI – 175

Dehli (India): AQI – 174

Katmandú (Nepal): AQI – 173

Mumbai (India): AQI – 173

Dhaka (Bangla Desh): AQI - 170

In US and Canada air pollution (AQI) in some major cities is as follows:

Detroit (United States): AQI – 86

Los Angeles (United States): AQI – 85

Toronto (Canada): AQI –  61

Vancouver (Canada): AQI – 48

Portland (United States): AQI – 45

In Europe air pollution (AQI) in some major cities is as follows:

Poznan (Poland): AQI - 155

Krakow (Poland): AQI – 154

Wroclaw (Poland): AQI – 132

Milano (Italy): AQI – 112

Budapest (Hungary): AQI – 105

Sarajevo (Bosnia-Herzegovina): AQI – 104

Bruselas (Belgium): AQI - 95

Lyon): AQI -  86

In Latin America air pollution (AQI) in some major cities is as follows:

Lima (Peru): AQI – 95

Mexico City (Mexico): AQI – 84

Santiago (Chile): AQI – 61

Sao Paulo (Brazil): AQI – 56

Bogotá (Colombia): AQI – 16

Parameters used to calculate the US Air Quality Index 

O3(ppb)	O3(ppb)	PM2.5(μg/m3)	PM10(μg/m3)	CO(ppm)	SO2(ppb)	NO2(ppb)	AQI	AQI
Clow - Chigh(avg)	Clow - Chigh(avg)	Clow- Chigh(avg)	Clow - Chigh(avg)	Clow - Chigh(avg)	Clow - Chigh(avg)	Clow - Chigh(avg)	Ilow- Ihigh	Category
0-54 (8-hr)	-	0.0-12.0 (24-hr)	0-54 (24-hr)	0.0-4.4 (8-hr)	0-35 (1-hr)	0-53 (1-hr)	0-50	Good
55-70 (8-hr)	-	12.1-35.4 (24-hr)	55-154 (24-hr)	4.5-9.4 (8-hr)	36-75 (1-hr)	54-100 (1-hr)	51-100	Moderate
71-85 (8-hr)	125-164 (1-hr)	35.5-55.4 (24-hr)	155-254 (24-hr)	9.5-12.4 (8-hr)	76-185 (1-hr)	101-360 (1-hr)	101-150	Unhealthy for Sensitive Groups
86-105 (8-hr)	165-204 (1-hr)	55.5-150.4 (24-hr)	255-354 (24-hr)	12.5-15.4 (8-hr)	186-304 (1-hr)	361-649 (1-hr)	151-200	Unhealthy
106-200 (8-hr)	205-404 (1-hr)	150.5-250.4 (24-hr)	355-424 (24-hr)	15.5-30.4 (8-hr)	305-604 (24-hr)	650-1249 (1-hr)	201-300	Very Unhealthy
-	405-504 (1-hr)	250.5-350.4 (24-hr)	425-504 (24-hr)	30.5-40.4 (8-hr)	605-804 (24-hr)	1250-1649 (1-hr)	301-400	Hazardous
-	505-604 (1-hr)	350.5-500.4 (24-hr)	505-604 (24-hr)	40.5-50.4 (8-hr)	805-1004 (24-hr)	1650-2049 (1-hr)	401-500

Air Quality Index health implications

AQI	Air Pollution Level	Air Pollution Category	Health Implications	Recommended Precautions
0–50	Level 1	Excellent(好极了)	No health implications.	Everyone can continue their outdoor activities normally.
51–100	Level 2	Good(良好)	Some pollutants may slightly affect very few hypersensitive individuals.	Only very few hypersensitive people should reduce outdoor activities.
101–150	Level 3	Lightly Polluted(輕度汚染)	Healthy people may experience slight irritations and sensitive individuals will be slightly affected to a larger extent.	Children, seniors and individuals with respiratory or heart diseases should reduce sustained and high-intensity outdoor exercises.
151–200	Level 4	Moderately Polluted(中度汚染)	Sensitive individuals will experience more serious conditions. The hearts and respiratory systems of healthy people may be affected.	Children, seniors and individuals with respiratory or heart diseases should avoid sustained and high-intensity outdoor exercises. General population should moderately reduce outdoor activities.
201–300	Level 5	Heavily Polluted(重度汚染)	Healthy people will commonly show symptoms. People with respiratory or heart diseases will be significantly affected and will experience reduced endurance in activities.	Children, seniors and individuals with heart or lung diseases should stay indoors and avoid outdoor activities. General population should reduce outdoor activities.
>300	Level 6	Severely Polluted(嚴重)	Healthy people will experience reduced endurance in activities and may also show noticeably strong symptoms. Other illnesses may be triggered in healthy people. Elders and the sick should remain indoors and avoid exercise. Healthy individuals should avoid outdoor activities.	Children, seniors and the sick should stay indoors and avoid physical exertion. General population should avoid outdoor activities.

Note 1: PM10 (particles smaller than 10 microns) and PM2.5 (particles smaller than 2.5 microns, about 25 to 100 times thinner than a human hair).

Note 2: the average for Uruguay is 11 μg/m3