Substantial changes in Gaseous pollutants and health effects during COVID-19 in Delhi, India

Hazard identification

Hazard identification is a procedure to identify the pollutant in an ambient environment that is likely to induce harmful effects on human health (Saliba et al., 2016). The identification of gaseous pollutants such as SO₂, NO₂, CO, and O₃ that pose a risk to human health was performed through a review of existing literature (United States Environmental Protection Agency, 2009; Gilbert et al., 2019; Kim, Kang & Kim, 2018; Odekanle et al., 2020).

Exposure assessment

The exposure assessment calculates the population exposed to the pollutant, the magnitude, and the duration of exposure. In the present study, the inhalation route is the major route of exposure to the identified pollutants. We estimated the daily and annual readings for normal and acute exposure periods for different age groups, namely infants (birth to a year), toddlers (2–5 years), children (6–12 years), and adults (19–75 years). The classification of age groups has been carried out according to Morakinyo et al. (2017, 2020).

For exposure to non-carcinogenic pollutants (SO₂, NO₂, and O₃), the acute exposure rate equation is mentioned as: (1)${\displaystyle \text{AHD}=C\times \text{IR}/\text{BW}}$ where AHD is the average hourly dose for inhalation (µg/kg/hour), C is the concentration of the pollutant (µg/m³), IR is the inhalation rate (m³/h), and BW is the body weight (kg) (World Health Organization, 1999). For exposure to non-carcinogenic pollutants (NO₂, SO₂, O₃), the chronic exposure equation used for the inhalation exposure route is: (2)${\displaystyle \text{ADD}=\left(\text{C}\times \text{IR}\times \text{ED}\right)/\left(\text{BW}\times \text{AT}\right)}$ where, ADD is the average daily dose of the pollutant (µg/kg/day), ED is the exposure duration (days), and AT is the averaging time (days) (USEPA, 1997). (3)${\displaystyle \text{ED (exposure duration)}=\text{ET}\times \text{EF}\times \text{DE}}$ where, ET is the exposure time (hour/day), EF is the exposure frequency (days/year), and DE is the duration of exposure (year). The recommended values in equations of the daily exposure dose of NO₂, SO₂, and O₃ are presented in Table 1.

Risk characterization

Risk characterization is an estimation of risk to human health due to exposure to a pollutant. In this study, the health risk estimate of possible non-carcinogenic exposure to the pollutant was determined using the hazard quotient (HQ) method (Muller et al., 2003; Saliba et al., 2016).

The risk to health due to exposure to CO, NO₂, SO₂, and O₃ through the inhalation route was estimated using Eqs. (4) and (5) (4)${\displaystyle \text{ADD}inh=\frac{C\times \text{InhR}\times \text{EF}\times \text{ED}}{\text{BW}\times \text{AT}}}$ ${\displaystyle \text{HQ}=\frac{\text{ADD}}{\text{REF}}}$

or (5)${\displaystyle \text{HQ}=x=\frac{\mathrm{AHD}}{\mathrm{REF}}\text{(acute exposure)}}$

REF is the reference exposure level for an exposed pollutant that has significant adverse health effects that are likely to occur in exposed groups compared to the unexposed group. Therefore, the RELs for the different pollutants are prescribed by NAAQS. For NO2 and SO2, these are 188 µg/m³ and 125 µg/m³ for daily and 40 µg/m³,50 µg/m³ for annual exposure besides 120 µg/m³ O₃ of 8 h duration (Morakinyo, Mukhola & Mokgobu, 2020).

New Delhi (ND)

The monitoring station of RK Puram is represented as the New Delhi zone. The average concentration of RK Puram station was 18.91, 34.79, 34.03, 31.09, and 22.61 µg/m³ for NO, NO₂, NOx, O₃, and NH_3, respectively (Fig. 3A). The average concentrations for NO, NO₂, NOx, O₃, and NH₃ were calculated to be 42.62, 46.06, 59.08, 24.59, and 17.10 µg/m³ for pre-lockdown whereas they were 3.58, 18.71, 12.85, 55.53 and 19.99 µg/m³ for lockdown I, 3.56, 22.15, 14.64, and 28.16 µg/m³ for lockdown II, 2.11, 20.92, 12.85, 58.05 and 31.28 µg/m³ for lockdown III, whereas 4.78, 38.52, 24.39, 66.39, and 19.24 µg/m³ for lockdown IV, respectively. The average concentrations for unlockdown I, unlockdown II, unlockdown III, and unlockdown IV were observed to be 4.42, 7.36, 10.86, 21.06 µg/m³ for NO, 28.70, 32.89, 30.40, 38.99 µg/m³ for NO2, 18.94, 23.43, 30.40, 38.99 µg/m³ for NOx, whereas, 24.75, 35.18, 20.24, 21.60 µg/m³ for NH3, respectively. The average values of ozone were found to be 19.35, 10.79, 28.25, and 31.38 µg/m^3, respectively. The level of gaseous pollutants witnessed a sharp decline during the first two of the four phases of lockdown corresponding to the last year, except for the ozone level. Further, the role of meteorological parameters, especially rainfall, has been considered a significant factor in reducing the concentration and washing out of various atmospheric pollutants (Elperin et al., 2011; Guo et al., 2016; Bedi et al., 2020)

South Delhi (SD)

Karni Singh Marg, Jawaharlal Nehru Stadium, Lodhi Road, and Sri Aurobindo Marg were four monitoring stations under the South Delhi zone. The average concentrations were found to be 20.59 µg/m³ (NO), 27.47 µg/m³ (NO2), 34.49 µg/m³ (NOx), 46.79 µg/m³ (O₃), 27.45 µg/m³ (_NH3), and 12.22 µg/m³ (SO₂), respectively (Fig. 3B). The average concentrations for NO, NO₂, NOx, O₃, NH_3, andSO₂ were calculated to be 10.48, 28.27, 23.57, 62.46, 23.57, and 11.29 µg/m³ for Karni Singh Marg, respectively. The average concentrations for NO, NO₂, NO_x, O₃, NH_3, andSO₂ were found to be 24.70, 37.74, 41.04, 28.33, and 18.01 µg/m³ for Jawaharlal Nehru Stadium, respectively, whereas for Lodhi road, it was 41.25, 18.54, 56.93, and 30.53 µg/m³ for NO, NO₂, NOx, and O₃ respectively. The average trend for all South Delhi zone monitoring stations was observed in Lodhi road >  JLN Stadium > Karni Singh Marg > Sri Aurobindo Marg for NO, NO_2, and NO_x during the pandemic situation, respectively. In the case of ozone, the trend was shown as Karni Singh Marg > JLN Stadium > Lodhi road > Sri Aurobindo Marg during the lockdown period, respectively. The average ozone level trend was observed to increase with subsequent lockdowns: lockdown I > lockdown II > lockdown III > lockdown IV, respectively. In contrast with the absence of dissociation factors (NO and NO2) in the ambient atmosphere, ozone levels at all stations showed a slightly increasing trend during the lockdown period, similar to those during the pre-lockdown period (Dhaka et al., 2020). The ozone level was higher at Karni Singh Marg than at other stations in South Delhi, possibly due to more restrictions on transport activities on these premises.

Northwest Delhi (NWD)

The monitoring stations such as Rohini, Ashok Vihar, and DTU were considered under the North-west Delhi. The average values for Rohini, Ashok Vihar, and DTU were 9.11 µg/m³ (NO), 27.06 µg/m³ (NO₂), 21.60 µg/m³ (NOx), 56.86 µg/m³ (O₃), 18.98 µg/m³ (NH₃), and 28.69 µg/m³ (SO₂) respectively, a similar result was reported in Delhi (Dutta & Jinsart, 2020) (Fig. 3C). The average trend of gaseous pollutants was found to be Rohini > DTU > Ashok Vihar, respectively. The average trend for gaseous pollutants at Rohini declined during the lockdown period and again increased continuously in the unlock-down period, except for O₃. The average concentration of O₃ was continuously increased with values 38.57, 80.75, 98.19, 110.88 µg/m³ during the pre-lockdown period, lockdown I, lockdown II, lockdown III, and lockdown IV, respectively. The trend of mean values of gaseous pollutants for Rohini was found to be O₃ > NO₂ > NH₃ > NO_x > SO₂ > NO. A similar trend was observed for DTU and Ashok Vihar, respectively. North-west Delhi zone is considered as the industrial zone where the decrease in NO₂ concentration could be attributed to the substantial reduction in emissions, mainly from transport sectors and industrial processes.

Northeast Delhi (NWD)

The monitoring station of ITO has been represented under the Northeast Delhi zone. The average concentrations of gaseous pollutants were calculated to be 39.53, 31.38, 53.52, 28.12, 24.75, and 9.78 µg/m³ for NO, NO₂, NOx, O₃, NH₃, and SO₂, respectively (Fig. 3D). The average trend of gaseous pollutants for ITO observed a sharp decline in lockdown I > lockdown II > l lockdown III > lockdown IV during the lockdown period. In contrast, the unlockdown period showed an increasing trend in the order of unlockdown I >  unlockdown II > unlockdown III > unlockdown IV, respectively, except O₃. Ozone levels during the lockdown period witnessed an increase as a result of lowering the concentration of NOx, which may result in ozone intensification levels in urban areas (Mahato, Pal & Ghosh, 2020). In addition, a natural increase in insolation and temperature from March to August in the northern hemisphere might lead to a rise in the value of ozone when the maximum ozone concentration is usually recorded (Gorai, Tchounwou & Mitra, 2017).

North Delhi zone (NoD)

The monitoring stations such as Alipur, Narela, Bawana, Wazirpur, and North Campus were considered under the North Delhi zone. The average concentrations for gaseous pollutants were calculated to be 15.51, 28.43, 29.44, 27.83, 30.91, and 14.53 µg/m³ for NO, NO₂, NOx, O₃, NH_3, and SO₂, for the North Delhi zone, respectively (Fig. 3E). The average values for NO, NO₂, NOx, O₃, and NH₃ for Alipur were observed to be 3.26, 23.44, 15.13, 52.65, and 20.51µg/m³ during the lockdown period, whereas 5.05, 14.09, 11.37, 17.34, 26.66 µg/m³ during the unlockdown period. The level of pollutants slightly increased during the unlockdown phase, except for ozone; Dutta & Jinsart (2020) noticed a similar result in Delhi. The average values for NO, NO₂, NOx, O₃, and NH₃ for Narela were observed to be 4.17, 31.13, 19.39, 70.73, 34.99, and 15.86 µg/m³ during the lockdown period, whereas 8.35, 27.73, 21.55, 21.06, 17.06, and 5.86 µg/m³ during the unlockdown period. The average concentrations for NO, NO₂, NOx, O₃, NH_3, and SO₂ for Wazirpur were found to be 8.21, 33.32, 24.33, 32.31, 30.01, and 15.72 µg/m³, whereas 21.52, 32.16, 22.85, 13.43, 35.67, 19.80 µg/m³ during the unlockdown period, respectively. The average concentrations for NO, NO₂, NOx, O₃, and SO₂ for Bawana were 1.04, 22.2, 12.7, 76, and 22.5 µg/m³, whereas 2.69, 22.5, 14.2, 34.1, 7.87 µg/m³ during the unlockdown period, respectively. The average concentrations for NO, NO₂, NOx, and O₃, for North Campus University were reported as 10.69, 17.22, 27.91, and 18.81 µg/m³ during the lockdown period and 17.88, 16.73, 33.13, and 32.82 µg/m³ during the unlockdown period, respectively. The results for gaseous pollutants witnessed a sharp decline during the lockdown period, whereas it slightly increased during the unlockdown period. A similar study conducted in China reported a 6.76 and 24.67 percent reduction in SO₂ and NO₂ (Dantas et al., 2020).

Southeast Delhi (SED)

The monitoring stations of Okhla and Siri Fort have been represented under the South Delhi zone. The average concentrations for Vivek Vihar were observed as 11.02, 27.08, 33.93, 36.87, 28.09, 24.60 µg/m³ whereas, for Siri Fort, it was 13.34, 31.16, 28.32, 65.30, 19.56, 8.23 µg/m³ for NO, NO₂, NOx, O₃, NH_3, and SO₂ during the pandemic situation respectively (Fig. 3F). The decline in NO, NO₂, and NOx could be mainly attributed to restrictions imposed on transport (public and private vehicular movement) during the lockdown. The mean trend for NO, NO₂, NOx, O₃, NH_3, and SO₂ was found to be lockdown III > lockdown IV > lockdown I > lockdown II>during the lockdown period, whereas for the unlock-down period, it showed an increasing trend in the order of unlockdown IV > unlockdown II > unlockdown III > unlockdown I respectively except for O₃.

East Delhi (ED)

The monitoring stations of Arya Nagar, Vivek Vihar, Patparganj, and Anand Vihar have been represented in East Delhi. The average concentrations of East Delhi were found to be 13.24, 22.43, 30.19, 37.99, 37.99, 36.21, and 26.21 µg/m³ for NO, NO₂, NOx, O₃, NH_3, andSO₂ during the pandemic period, respectively (Fig. 3G). The mean trend for NO, NO₂, NOx, O₃, NH_3, and SO₂ was found to be lockdown IV > lockdown III >  lockdown II > lockdown I during the lockdown period, whereas, for the unlockdown period, it observed an increasing trend in the order of unlockdown IV > unlockdown II > unlockdown III > unlockdown I respectively. Anand Vihar has been considered one of the inter bus terminals of NCR, which found massive traffic congestion and slow movement. During the unlock-down period, the state government has announced relief measures for migrants, and arrangements for buses for migrants to return to their native place increased the level of pollution at Anand Vihar station (Singh & Kumar, 2021).

Southwest Delhi (SWD)

Two monitoring stations of Najafgarh and Sri Aurobindo Marg have been represented in Southwest Delhi. The average concentrations were found to be 2.40, 9.04, 12.47, 12.08, 33.57, 19.24 µg/m³ for NO, NO₂, NOx, O₃, NH_3, and SO₂ during the pandemic period, respectively (Fig. 3H). The mean trend for NO, NO₂, NOx, O₃, NH_3, and SO₂ was found to be lockdown I >  lockdown II > lockdown II > lockdown IV during the lockdown period, whereas for the unlockdown period, it observed an increasing trend in the order of unlockdown IV >  unlockdown III > unlockdown II > unlockdown I respectively. The lowest concentrations for all gaseous pollutants were observed at Sri Aurobindo Marg due to more greenery and complete restriction of anthropogenic activities such as transportation, traveling, and industrial activities, which are the primary source of such pollutants (Sharma et al., 2020). Other studies conducted in different cities of India indicated that, on average, NO_x concentrations declined by 60%, further suggesting that the impact of lockdown on air quality was not homogenous across the country due to fluctuating background contributions (Kumari & Toshniwal, 2020; Gulia et al., 2021).