Elevated Levels of the Pollutant PM2.5 in Crowded Subways of Cities with High COVID-19 Related Mortality-Has COVID-19 Gone Underground?

Research Article

J Endocr Disord. 2021; 7(1): 1043.

Elevated Levels of the Pollutant PM2.5 in Crowded Subways of Cities with High COVID-19 Related Mortality-Has COVID-19 Gone Underground?

Baron YM*

University of Malta Medical School, Mater Dei Hospital, Malta

*Corresponding author: Yves Muscat Baron, University of Malta Medical School, Mater Dei Hospital, Msida, Malta

Received: December 24, 2020; Accepted: January 19, 2021; Published: January 26, 2021


COVID-19; Mortality; PM2.5; Subway/underground


A nationwide study by [1] in the United States indicated a connection between long-term exposure to particulate matter PM2.5 and COVID-19 related mortality [1]. COVID-19 had been found to be adherent to particulate matter and it has been suggested that PM2.5 is an airborne co-factor in COVID-19 infection [2]. PM2.5 minimum and maximum levels in subways have been found several times higher than surface level. An elevated mean PM2.5 concentration of 381 μg/m³ was reported on the Victoria Underground, approximately 16 times higher than surface PM2.5 results [3]. The World Health Organization guideline recommends that PM2.5 levels should not exceed an average level of 25 μg/m³ 24-hour. The guideline stipulates that a PM2.5 concentration of 10 μg/m³/24 hr is the lowest level at which combined cardiopulmonary and lung cancer mortality have been shown to rise in response to chronic exposure to PM2.5 [4]. Commuter use of underground rail is significant. On a global scale the use of transport via subways represents around 11% of worldwide public transport. In New York, the daily ridership is 5 million and decreased by 90% after the institution of the New York lockdown [5]. More than 200 million passengers commute daily in the 156 underground networks around the world. With the movement of rural populations to urban areas as in China, the ridership is increasing exponentially as in subways of Beijing and Shanghai.


The mortality rate from COVID-19 related deaths was obtained from 18 cities. Due to variable rates of COVID-19 testing and diagnoses between countries and cities, mortality as a marker for COVID-19 incidence was used as it is a more robust indicator of the occurrence of COVID-19 infection. Cities were selected as opposed to nations, because substantial regional differences in infection rates have noted within the same countries. One group of these cities did not have an elevated percentage of the population that succumbed to COVID-19 deaths. This group consisted, of Tokyo, Naples, Sydney Vienna, Hong Kong, Seoul, Toronto, Athens and Taipei. The second group had significantly higher COVID-19 mortality rates and included Tehran, Barcelona, Stockholm, Sao Paolo, Wuhan, Paris, Milan, London and in New York. The mortality rates were obtained from various sources including the John Hopkins. Corona Virus Resource Centre and the WHO Coronavirus website. The cut-off for mortality rates between the cities was taken 0.05%. There was a ten fold difference in the average mortality of both groups of cities. The maximum and minimum levels particulate matter PM2.5, were obtained for both groups of city subways from the literature pertaining to PM2.5 measurement. An important caveat is that measurements between stations from the same underground networks due to the age of the subway, the depth and length of the subway and the mode of its ventilation. Variation to PM2.5 also occurs with the network ridership, relative humidity and seasonal changes may also occur. Data regarding the underground networks of the city subways assessed were obtained from an internet site. The data retrieved included the number of stations, the length of the subway and the annual ridership for all networks.


The percentage mortality of the individual’s cities’ population related to COVID-19 infection correlated significantly with the PM2.5 levels on the subway platforms. The correlation of the percentage population mortality with PM2.5 was significant for minimum PM2.5 levels (p<0.01) and highly significant for maximum (p<0.00001) levels of PM2.5 (Table 1). The PM2.5 levels differed significantly between both groups of cities differentiated by COVID-19 related mortality. The cities’ subways with low COVID-19 death rates had minimum platform PM2.5 levels of 27.4 (SD+/-17.2 μg/m³) compared to 63.4 μg/m³ (SD+/-10.8 μg/m³) in cities with high COVID-19 associated mortality rates (p<0.01). Underground networks’ maximum levels of PM2.5 in cities with low COVID-19 mortality was 53.4 μg/ m³ (SD+/-21.8 μg/m³) while that of subways with high COVID-19 mortality had maximum platform PM2.5 levels of 172.1 μg/m³ (SD+/-98 μg/m³) (p<0.001) (Tables 1&3). The length of the subway network and number of stations was significantly different between both groups of cities. The cities with higher COVID-19 death rates had longer networks 230km (SD+/-111 km) versus 119 km (SD+/- 99 km) ( p<0.03). Similarly cities with higher COVID-19 mortality rates had more stations 191 (SD+/-109) versus 102 (SD+/-94), showing significance (p<0.047). Although the annual ridership in the cities with the high COVID-19 mortality was higher (1034x106 versus 751x106), this did not achieve statistical significance (Tables 2&4). The maximum PM2.5 correlated with the number of stations (p<0.045) and the length of the networks (p<0.044). The minimum PM2.5 did not achieve similar significant correlations. Ridership significantly correlated with number of stations (p<0.01) and the length of the network (p<0.02).