4  PM2.5 assessment before–during quarantine
This research seeks to evaluate the behavior of the main most polluting cities in the world from the comparison between a typical week, before quarantine (BQut), (considered in this document a week measured before entering confinement), and an atypical week, during quarantine (Qut) by COVID-19. This week (Qut) is considered for analysis, due to the restriction of most economic activities that involve reducing the carbon footprint. As an example, we have restrictive measures regarding citizen mobility and, therefore, vehicle mobility and industrial production are reduced.
These dates were selected according to Table 1, with a study range of one month before and after the start of Qut. The PM2.5 values are obtained from the online platform (WAQ Index, 2020) and official weather stations of each city, the analyzed information depends on obtaining or recording data from the study date. Cases are presented where meteorological stations that record PM2.5 are not obtained but only PM10, therefore, these cities could not be analyzed. Likewise, there are days of the weeks that present alterations due to atmospheric conditions in each city, therefore, no data was taken from these days. For 50 analized capital cities, 20% do not record PM2.5 data, corresponding to capitals from the countries of: Germany, Philippines, Romania and Bulgaria. On the other hand, for capitals with PM2.5 data, 45% of capitals correspond to the continent of Asia, followed by 42% to the continent of Europe, 10% to the continent of America and the remaining 25% to the continent of Africa. The air pollution level, health implications and cautionary statement according to the PM2.5 conditions can be seen in (WAQ Index, 2020).
Likewise, in the analyzed countries presented below, decreases, increases or a constant level of PM2.5 emissions are observed during confinement. This can be attributed to the confinement level in each country, at the beginning of the quarantine, or to the electricity generation increase depending on the generation technology used. It is important to clarify that not all countries indicate reductions in their PM2.5 particles emitted during the analyzed week (Qut); each capital reflects special conditions that must be analyzed individually and this is not the aim of this paper.

4.1  Asia
From Fig. 1, Fig. 2 , the most polluting capitals of Asia are presented. At a general level, 27% of the capitals had a tendency to decrease the PM2.5 emission during Qut, however, the cities Kathmandu, Hanoi, Jakarta, Singapore and Tokyo had a tendency to increase the PM2.5 concentration by 11%. It should be noted that Tokyo, with the exception of the other cities, did not present a mandatory quarantine but as an option of the Government, they requested non-mandatory teleworking. For the analyzed Asian capitals, Dhaka in typical times (BQut), is the capital with the highest PM2.5 particles pollutant, registering a weekly average of 183 μg/m3. This capital city, presents a reduction of 24% in the Qut period. Delhi, with a weekly average of 140 μg/m3, has the highest reduction in particles polluting compared to other Asian countries, presenting a 40% reduction during Qut week. Capital cities with an average concentration of 121.91 μg/m3 (Kabul, Ullabantar and Kuwait City) show average reductions of 33% BQut. However, Bejing shows a lower reduction of 8%. Capital cities with an average concentration of 106.83 μg/m3 (Kabul, Colombo and Tashkent) show average reductions of 28%. Tehran has a typical week concentration of 90 μg/m3. However, its reduction during the quarantine week was 39%. Finally, Astana, with an average weekly concentration of 61.25 μg/m3, reduced its concentration by 18% during the Qut season.
Fig. 1 Capital cities Ankara, Astana, Bangkok, Beijing, Colombo, Delhi, Dhaka, Kabul and Hanoi, PM2.5 levels before and during quarantine (WAQ Index, 2020).
Fig. 2 Capital cities Jakarta, Kathmandu, Kubait city, Ulaanbaatar, Tashkent, Tehran, Tel Aviv, Tokio and Singapore PM2.5 levels before and during quarantine (WAQ Index, 2020).

4.2  Europe
From Fig. 3, Fig. 4 it can be seen the seventeen (17) European capitals analyzed. In general, European capital cities in typical weeks record PM2.5 concentrations below 80 μg/m3, with an AQI between Good and Moderate. 50% of the studied European capitals, during the Qut had a tendency to decrease the PM2.5 concentration at an average of 23%. However, the other 50% show an increase in the confinement season. Budapest, with an annual average concentration of 48 μg/m3, is a city that presents an increase of 35%. This capital does not present total confinement, they are allowed to exercise, go to work, with measures that allow them to circulate more widely, going from having a good to moderate AQI during BQut. Bratislava, does not apply quarantine, leaving citizens to walk freely and exercise. However, the date presented in Table 1, there are distance restrictions, suspension of classes and closure of sectors such as hostelry. This European capital presents an increase in the Qut week, which went from a good to moderate AQI level. Paris, London, Vienna, Brussels and Prague, are capitals that generally have a good AQI level, with an average record of 31 μg/m3. However, in the confinement season, an increase in the concentrations of the PM2.5 particulate matter is observed.
Fig. 3 Capital cities Belgrade, Bern, Bratislavia, Brussels, Budapest, Cyprus, Kyiv, Lisbon and London PM2.5 levels before and during quarantine (WAQ Index, 2020).
Fig. 4 Capital cities Paris, Prague, Pristina, Sarajevo, Skopje, Vienna, Vilnius and Warsaw PM2.5 levels before and during quarantine (WAQ Index, 2020).

4.3  America and Africa
Regarding the American Continent, four (4) capitals are analyzed in this research. These capitals during typical periods, BQut, present a moderate AQI level, where Bogota is the capital with the highest PM2.5 concentration with an average of 98 μg/m3. Followed by, Mexico City with 74 μg/m3, Santiago de Chile with 68 μg/m3. Finally, Lima registers an average concentration of 58 μg/m3. As well as, the period in confinement in Bogota, presents the greatest decrease in the cities analyzed during the Qut, with a 57% PM2.5 reduction. Santiago de Chile, does not present quarantine in the entire city but in seven specific communes, which registers a 10% reduction in the concentration of this particulate matter. Mexico City presents a voluntary quarantine by the population without government restrictions, however a 2% reduction is observed during the study week in Qut. (Fig. 5 ).
Fig. 5 Capital cities Bogotá, Lima, Mexico City, Santiago de Chile and Kampala (Africa) PM2.5 levels before and during quarantine (WAQ Index, 2020).
Kampala, as the only registered capital with a high AQI level, registers a weekly average of 146 μg/m3 during the study week BQut. During the quarantine season and with the prohibition of vehicles and the closure of all stores except food, the PM2.5 concentration was reduced by 35%, going to a moderate AQI level.