Researchers selected Milan, Italy to study how air pollution may have changed as a result of COVID-19 lockdowns. Prior to lockdown (February 7 - February 20, 2020): 
- Average PM10 levels were 47.1 – 56.8  μg/m3 
- Average PM2.5 levels were ~40 μg/m3
- Average BC levels were ~4 μg/m3 
- Average benzene levels were ~2.25 μg/m3 
- Average CO levels were ~1.3 μg/m3
- Average SO2 levels were ~5 μg/m3 
- From 2016-2018, average NO2 levels were 50-60 μg/m3 in February and 45-50 μg/m3 in March 
- From 2016-2018, average O3 levels were 10-20 μg/m3 in February and 30-40 μg/m3 in March 

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Humans contribute to air pollution by releasing harmful substances into the earth's atmosphere. Over many decades, harmful substances have accumulated to produce areas with severely polluted air. Polluted air impacts human health in many ways. 
- High NO2 levels irritate the respiratory system 
- High BC (black carbon) levels are associated with respiratory and cardiovascular disease. BC may also be represented by BCff (fossil fuels) or BCbb (biomass burning) 
- PM10 and PM2.5 both measure fine particulates in the air, which irritate the respiratory system
- O3 (ozone) in exceedingly high concentrations causes airway inflammation and may reduce lung function 
- High outdoor CO levels may reduce oxygen transport to the heart 
- High THC (total hydrocarbons) and high NMHC (non-methane hydrocarbons) levels are toxic and may cause cancer 
- Low formaldehyde levels are enough to irritate the respiratory tract. Prolonged exposure may cause cancer 
- High AOD levels (>1) indicate heavy pollutants in the air 
- High AQI values (100-500) indicate poor air quality conditions (ranging from unhealthy for sensitive groups to hazardous)  
- BTEX values (benzene, toluene, ethylbenzene, xylene) indicate levels of hazardous and often carcinogenic compounds dangerous to human health 

Air Pollution (Pre-COVID-19)

Cristina Collivignarelli, M., Abbà, A., Bertanza, G., Pedrazzani, R., Ricciardi, P., & Carnevale Miino, M. (2020). Lockdown for CoViD-2019 in Milan: What are the Effects on Air Quality? Science of the Total Environment, 732. https://doi.org/10.1016/j.scitotenv.2020.139280 

Lockdown for CoViD-2019 in Milan: What are the Effects on Air Quality?

Researchers selected California, USA to study how air pollution changed during the COVID-19 outbreak. Prior to the outbreak, California, USA: 
- had high levels of air pollution, with five cities having the worst pollution from particulate matter in the country  
- had generally high levels of O3, PM2.5, PM10, NO2, and CO. Data shows these levels remained fairly constant through March of 2015-2019 with no sharp decreases 

Air Pollution: California (USA) (Pre-COVID-19) 

Researchers selected Ahmedabad, India to study how air pollution changed during the COVID-19 outbreak. Prior to the outbreak, Ahmedabad: 
- had PM2.5 levels as high as 100 μg/m3 
- had PM10 levels as high as 150 μg/m3 
- had NO2 levels as high as 100 μg/m3

Air Pollution: Ahmedabad (India) (Pre-COVID-19)

Researchers selected South Korea to study how air pollution changed during the COVID-19 outbreak. Prior to the outbreak, based on data from December-April in the years 2017-2019, Korea: 
- had daily average PM2.5 levels recorded for each year at 34.23 μg/m3, 28.65 μg/m3, and 37.37 μg/m3
- had daily average PM10 levels recorded for each year at 54.67 μg/m3, 46.73 μg/m3, and 60.77 μg/m3  
- had daily average NO2 levels recorded for each year at 23.61 ppb, 20.31 ppb, and 20.38 ppb 
- had daily average CO levels recorded for each year at 0.536 ppm, 0.495 ppm, and 0.513 ppm 

Air Pollution: South Korea (Pre-COVID-19)

Researchers selected Barcelona, Spain to study how air pollution changed during the COVID-19 outbreak. Prior to the outbreak, Barcelona: 
- had markedly high levels of NO2 and BC in the air
- measured at a high level of PM10
- had too little O3 

Air Pollution: Barcelona (Spain) (Pre-COVID-19)

Researchers selected Sigrauli, India to study how air pollution changed during the COVID-19 outbreak. Prior to the outbreak, Sigrauli:
- had dangerously high levels of PM10 with recorded values as high as 250 μg/m3
- had dangerously high levels of PM2.5 with recorded values as high as 100 μg/m3
- had moderately high levels of NO2 with recorded values as high as 75 μg/m3
- had dangerously high levels of SO2 with recorded values as high as 160 μg/m3 

Air Pollution: Sigrauli (India) (Pre-COVID-19)

Researchers selected Mumbai, India to study how air pollution changed during the COVID-19 outbreak. Prior to the outbreak, Mumbai: 
- had moderately high levels of PM10 with recorded values as high as 150 μg/m3
- had slightly high levels of PM2.5 with recorded values as high as 40 μg/m3
- had moderately high levels of NO2 with recorded values as high as 80 μg/m3
- had low levels of SO2 averaging around 20 μg/m3

Air Pollution: Mumbai (India) (Pre-COVID-19)

Researchers selected Rio de Janeiro, Brazil to study how air pollution changed during the COVID-19 outbreak. Prior to the outbreak, Rio de Janeiro: 
- was highly polluted by PM10s and O3 
- had some pollution from NO2, CO, THC, and NMHC 

Air Pollution: Rio de Janeiro (Brazil) (Pre-COVID-19)

Researchers selected Malaysia to study how air pollution changed during the 2020 Malaysia Movement Control Order (MCO), which kept the country under a temporary lock down during the COVID-19 outbreak. Prior to the lock down, Malaysia: 
- had daily PM2.5 levels as high as 69.2 μg/m3, compared to a standard limit of 35 μg/m3 
- had consistently high PM2.5 levels at multiple air quality monitoring stations throughout the country 

Air Pollution: Malaysia (Pre-COVID-19)

Researchers selected Delhi, India to study how air pollution changed during the COVID-19 outbreak. Prior to the outbreak, Delhi: 
- had dangerously high levels of PM10 with recorded values as high as 300 μg/m3 
- had dangerously high levels of PM2.5 with recorded values as high as 100 μg/m3 
- had moderately high levels of NO2 with recorded values as high as 80 μg/m3  
- had low levels of SO2 averaging around 15 μg/m3

Air Pollution: Delhi (India) (Pre-COVID-19)

Researchers selected East Asia to study how air pollution changed during the COVID-19 outbreak. Prior to the outbreak, in February 2019:
- average AOD levels were recorded at 1.26 for the Beijing-Tianjin-Hebei area, 1.63 for Wuhan, 0.68 for Seoul, and 0.48 for Tokyo 
- NO2 levels were recorded at 9.3E+15 molecules/cm2 for the Beijing-Tianjin-Hebei area, 1.5E+16 molecules/cm2 for Wuhan, 1.6E+16 molecules/cm2 for the Soul Metropolitan Area, and 9.8E+15 molecules/cm2 for the Tokyo Metropolitan Area 
- Formaldehyde levels were recorded at 7.4E+15 molecules/cm2 for the Beijing-Tianjin-Hebei area, 7.3E+15 molecules/cm2 for Wuhan, 5.8E+15 molecules/cm2 for Soul, and 3.9E+15 molecules/cm2 for Tokyo 
- SO2 levels were recorded at 1.3E+16 molecules/cm2 for the Beijing-Tianjin-Hebei area, 3.9E+15 molecules/cm2 for Wuhan, 	8.2E+15 molecules/cm2 for Soul, and 2.7E+15 molecules/cm2 for Tokyo 
- CO levels were recorded at 3.23E+18 molecules/cm2 for the Beijing-Tianjin-Hebei area, 3.51E+18 molecules/cm2 for Wuhan, 2.95E+18 molecules/cm2 for the Soul Metropolitan Area, and 2.51E+18 molecules/cm2 for the Tokyo Metropolitan Area 
- O3 levels were recorded at 48.5 µg/m3 for East China, 47.3 µg/m3 for the Beijing-Tianjin-Hebei area, and 37.1 µg/m3 for Wuhan 

Air Pollution: East Asia (Pre-COVID-19)

Researchers selected China to study how air pollution changed during the COVID-19 outbreak. Prior to the outbreak, China: 
- had very high NO2 levels, particularly in North China and Northeast China 
- had numerous major cities with poor air quality and pollution from PM2.5, PM10, SO2, CO, and O3 
- had notable declines in NO2 levels during the Lunar New Year (late January to late February), with levels increasing following conclusion of the holidays 

Air Pollution: China (Pre-COVID-19)

Researchers selected the Dwarka River Basin in India to study how air pollution changed during the COVID-19 outbreak. Prior to the outbreak, the Dwarka River Basin:
- had dangerously high levels of PM10 with recorded values as high as 278.79 μg/m3 

Air Pollution: Dwarka River Basin (India) (Pre-COVID-19)

Researchers selected Baghdad, Iraq to study how air pollution changed during the COVID-19 outbreak. Prior to the outbreak (January 16-February 29), Baghdad: 
- had average NO2 levels at 91 μg/m3
- had average O3 levels at 8 μg/m3 
- had average PM2.5 levels at 40 μg/m3 
- had average PM10 levels at 119 μg/m3 
- had an average AQI value of 120

Air Pollution: Baghdad (Iraq) (Pre-COVID-19)

Researchers selected Almaty, Kazakhstan to study how air pollution changed during the COVID-19 outbreak. Prior to the outbreak and subsequent lockdowns:
- average PM2.5 levels were recorded at 53 μg/m3 (February 21 - March 18, 2018) and 38 μg/m3 (March 19 - April 14, 2018)
- average PM2.5 levels were recorded at 66 μg/m3 (February 21 - March 18, 2019) and 40 μg/m3 (March 19 - April 14, 2019)
- In 2018, 23 days exceeded the WHO daily limit for PM2.5 and in 2019, 25 days exceeded this limit 
- BTEX values varied, likely caused by vehicle emissions and coal burning 
- From March 2 - March 18, 2020: average NO2 levels (37 μg/m3) average SO2 levels (49 μg/m3) average CO levels (674 μg/m3) and average O3 levels (30 μg/m3) 

Air Pollution: Almaty (Kazakhstan) (Pre-COVID-19)

Researchers selected Dhaka City, Bangladesh to study how air pollution may have changed as a result of COVID-19. Prior to the COVID-19 outbreak: 
- Average PM2.5 levels (80 μg/m3) were 5x higher than the Bangladeshi standard and 8x higher than the WHO standard 
- Air pollution was severe enough to cause various public health problems and accounted for an estimated 8.8 million premature deaths per year 
- Prior to COVID-19 lockdown (March 8 - March 25, 2020) average PM2.5 levels were 101.8 μg/m3, average O3 levels were 185.13 μg/m3, average CO levels were 13.96 μg/m3, average SO2 levels were 243.05 μg/m3, and average NO2 levels were 500.97 μg/m3. AQI values were recorded as high as 180

Air Pollution: Dhaka City (Bangladesh) (Pre-COVID-19)

Air Pollution: Milan (Italy) (Pre-COVID-19)

Researchers selected Salé City, Morocco to study how air pollution changed during the COVID-19 outbreak. The period March 11 - March 20, 2020 represented the pre-lockdown timeframe. 
- Average PM10 levels were 114.6 μg/m3
- Average NO2 levels were 5.6 μg/m3 
- Average SO2 levels were 6.6 μg/m3 

Air Pollution: Salé City (Morocco) (Pre-COVID-19) 

Researchers selected Ontario, Canada to study how air pollution changed during the COVID-19 outbreak. Data for pre-COVID-19 air quality was gathered from the first 17 weeks of years 2015-2020. 
- Average fine particulate matter levels were 7 μg/m3, with a max value of 76 μg/m3 
- Average NO2 levels were 9 ppb, with a max value of 78 ppb 
- Average levels of other nitrogen oxides was 11 ppb, with a max value of 387 ppb 
- Average ozone levels were 30 ppb, with a max value of 94 ppb 

Air Pollution: Ontario (Canada) (Pre-COVID-19)

Researchers selected New York City, USA to study how air pollution changed during the COVID-19 outbreak. Data from the first 17 weeks of years 2015-2019 was used to represent pre-COVID-19 air quality levels. 
- PM2.5 levels: 1.492 μg/m3 (2015) 1.120 μg/m3 (2016) 0.715 μg/m3 (2017) 0.651 (2018) 0.365 (2019) 
- NO2 levels: 4.216 ppb (2015) 1.940 ppb (2016) 2.122 ppb (2017) 1.943 ppb (2018) 1.267 ppb (2019) 
- General trends indicate air quality improved each subsequent year in regard to both PM2.5 and NO2 levels 

Air Pollution: New York City (USA) (Pre-COVID-19)

Researchers selected Shenyang, China to study how air pollution changed during the COVID-19 outbreak. Pre-lockdown data was recorded from January 1 - January 25, 2020. Data on ozone levels was gathered from 2018 and 2019. 
- In 2018, O3 levels were as high as 288 μg/m3. In 2019, O3 levels were as high as 259 μg/m3 
- Ozone levels had a negative correlation to concentrations of other pollutants in the pre-lockdown period 

Air Pollution: Shenyang (China) (Pre-COVID-19) 

Researchers selected Athens, Greece to study how air pollution changed during the COVID-19 pandemic. Pre-lockdown data was recorded from March 1, 2020 - March 22, 2020. 
- Average CO2 levels were 433.6 ppm 
- Average CO levels were 0.40 mg/m3 
- Average NO2 levels were 26.7 μg/m3 
- Average PM2.5 levels were 17.3 μg/m3 
- Average BC levels were 1.57 μg/m3
- Average BCff values were 1.02 μg/m3 

Air Pollution: Athens (Greece) (Pre-COVID-19) 

Researchers selected Paris, France to study how air pollution changed during the COVID-19 outbreak. Pre-lockdown data was gathered from 2015-2019. Pollutant levels were compared between periods March 17 - June 1 and January 1 - March 17. 
- In 2015, NO2 levels decreased 13% and PM2.5 levels decreased 19%
- In 2016, NO2 levels decreased 4% and PM2.5 levels decreased 13%
- In 2017, NO2 levels decreased 14% and PM2.5 levels decreased 41%
- In 2018, NO2 levels decreased 5% and PM2.5 levels decreased 6% 
- In 2019, NO2 levels decreased 8% and PM2.5 levels decreased 7% 

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