5  Results and discussions
The state of air quality is based on the environmental monitoring stations that are available in each city. These stations determine the hourly concentration of air pollutant particles, including PM2.5 and PM10 particles. According to the WHO (World Health Organization, 2018), air pollution represents a major environmental health risk. For this reason, it is so important to have air quality monitored in cities, essentially in capital cities, and in this case, it is more important to monitor PM2.5 due to the risks explained in the introduction. Therefore, it is essential to know how environmental control of capitals is, for this, it is essential to know the number of meteorological stations that measure PM2.5.

5.1  Air quality stations vs. population of the most polluted capitals cities
Fig. 6 presents the relationship between quantity of inhabitants per one (Pp) environmental station (ES) is presented. (Pp/ES) in each capital. It is reflected by a range of colors applied to each capital. The main cities where greater control of air quality is observed, measured according to the Pp/ES ratio, are registered in quartile 2 (Q2) and quartile 3 (Q3), with a dark and light blue color, the which are: i. Tel Aviv, Brussels and Pristina, these capitals present an average of 61,926 Pp/ES. ii. Bratislavia, Bern, Sarajevo, present a ratio of 126,317 Pp/ES. iii. Vienna, and Ulaanbaatar, 170,930 Pp/ES. iv. Budapest, Lisbon and Prague present an average ratio of 244,696 Pp/ES. Capitals that register between 300,000 Pp/ES and 500,000 Pp/ES, are registered between quartile (Q1) and quartile (Q3). In Q1, the capitals London and Bangkok are observed, which present populations and stations above the average.
Fig. 6 Population vs Environmental stations.
In Q3 are the capital cities Skopje, Kathmandu, Paris and Belgrade, who register a population and stations lower than average. The capitals with a registry between 500,000 Pp/ES and 700,000 Pp/ES are Bogota, Tehran, Tokyo, Vilnues, Kyiv, Warsaw, Kubait City, and Santiago. Finally, from 700,000 Pp/ES to 1,000,000 Pp/ES, are Colombo, Ankara, Mexico City and Cyprus. These capitals cities are identified between Q1 and Q3. In Q4 the capitals with a low number of stations measuring PM2.5 can be seen, due to the number of inhabitants of each one. Above 3,000,000 Jakarta, Hanoi, Lima and Dhaka are identified. Lima has several weather stations, however, there is only one station that PM2.5 register data. The cities with the highest pollution, Delhi (India) and Beijing (China), present a ratio of 1,766,333 Pp/ES and 1,227,059 Pp/ES respectively. Although meteorological stations are installed based on population, it is observed that countries such as Singapore, Hanoi, Kakarta and Dhaka with populations over 5,000,000, have a number of below-average meteorological stations, which makes monitoring difficult and not It allows to have data of the real contamination.
Likewise, it should be considered that the analysis of any study is oriented to an economic system that restricts the conditions of each country. In this case, when analyzed by continents and population, Europe with a GDP higher than the South American or Asian economies, presents a greater boost in reducing emissions of polluting particles. This, indicating that the continent has a good AQI index and an optimal Pp/ES ratio for environmental analysis. Their concern for the environment and compliance with the Kyoto protocol and the commitments to COP21 are reflected in their environmental indicators. This continent recognizes that air quality influences labor productivity, investments in healthcare expenses and improvement in quality of life, among others.
In contrast, the Asian continent, with an unfavorable view compared to other continents, it is observed that the Asian economies are the main contributors to environmental pollution, which is why it attributes to greater inequality and poverty. This allows linking to this study (Fig. 6) where a low update is observed in technologies that monitor the quality air in Asian cities, this reflected in the low Pp/Es ratio. This indicates the low commitment to improve the life quality and the low incentives to reduce the cost attributed by air pollution. In general, the most densified in population cities reflect PM2.5 contamination high with rates higher than 50 μg/m3.

5.2  Global analysis
Making a global balance of the analyzed countries, the variation of the PM2.5 concentration had an average reduction of 12%. The highest reduction occurs in the African continent with one (1) country analyzed (33%), followed by the American continent (22%) and the Asian continent (16%); finally, the European continent, in which a favorable reduction result is not generally observed (5%). Fig. 7 shows the PM2.5 variation in a global map of capital cities. The absolute value of the variation can be identified by size of the circle. In red we find the cities with increases in their PM2.5 measurement and in pink the cities with PM2.5 reduction, the city with the greatest PM2.5 reduction within the data collected is Bogota, with a reduction of 57% compared to a typical week.
Fig. 7 PM2.5 Reduction in quarantine week.
Fig. 8 shows in detail the PM2.5 quantitative variation of the analyzed cities, as well as its mean concentration. The gray color represents the PM2.5 behavior under typical conditions, and the light blue color represents the confinement mode. Dhaka, the most polluted capital of this particulate matter, had a 14% reduction. It is observed as the first seventeen (17) most contaminated capitals in the world, from Dhaka, to Skopje, exceed the PM2.5 average concentrations (75.78 μg/m3) from before quarantine, maintaining an AQI level from moderate to unhealthy. Likewise, during typical times twelve (12) countries present an AQI level below the quarantine average (Bern - Kyiv) almost all with a good AQI level. During the quarantine, sixteen (16) capitals exceed the PM2.5 registered average (66.92%), equivalent to a moderate AQI level, and nine (9) cities registered a good AQI level. Specifically, the three most polluted capital cities that are Dhaka, followed by Kampala and Delhi, reduced their PM2.5 concentration by 14%, 35% and 40% respectively. The capital city with the highest PM2.5 reduction during quarantine was Bogotá, with a percentage of 57%, going from a moderate to a good AQI level. Kubait City, presents the second largest PM2.5 reduction (42%). Finally, with reductions over 40%, there are the cities of Delhi and Tehran.
Fig. 8 Weekly average without quarantine vs weekly average with quarantine.
In Europe, the continent with the best environmental conditions in normal times, a very high increase in PM2.5 is observed in the cities of Prague, Vienna and Bratislava, cities very close to each other; apparently, an isolated effect produced by winds or fires.