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      Reduction in air pollution and attributable mortality due to COVID-19 lockdown

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          Abstract

          Lockdown measures reducing the transmission of COVID-19 have led to a temporal improvement in air quality worldwide. An assessment by Kai Chen and colleagues 1 reported that a drop in air pollution in 367 Chinese cities was associated with 8911 prevented deaths because of a reduction in nitrogen dioxide (NO2; 95% CI 6950–10 866) and 3214 deaths from a reduction in PM2·5 (95% CI 2340–4087). However, the method for quantifying reductions in air pollution and attributable mortality is on the basis of several simplifications and assumptions that are likely to bias the estimates. First, air pollution reductions estimated with the difference-in-difference approach cannot be directly attributed to the lockdown because this method does not cancel out the effect of weather on air pollution. To show this point, we estimated the change in NO2 during lockdown in 48 Spanish provinces, from March 15 to April 23, 2020, by estimating business-as-usual concentrations with a meteorology normalisation technique based on machine learning, 2 which shows the expected concentrations that would have occurred without lockdown. We then used exposure-response functions3, 4 to transform the daily observed and business-as-usual NO2 time series into daily NO2-attributable mortality time series, and the difference was compared with estimates obtained with the method in Chen and colleagues. 1 Our unbiased method increased the reduction in NO2 by 11% on average and increased the reduction in attributable deaths by 6·5%. Depending on how anomalous the weather was during lockdown and the reference periods in each of the Chinese cities, the bias could be even larger. Second, the baseline mortality used by the authors to quantify the temporal reduction in number of deaths is from 2018. The choice of this particular year is unjustified, and the use of data from one year can lead to biases in the estimates, particularly for cause-specific mortality. In addition, Chen and colleagues 1 used different sources of data for the calibration of historical exposure-response models and the estimations of the reduction in air pollution, which could lead to large biases because of inconsistent choice of air pollution stations. Finally, the authors state that “the COVID-19 outbreak led to improvements in air quality that brought health benefits in non-COVID-19 deaths”. We would like to raise a note of caution, given that some of the prevented deaths might correspond to the oldest and frailest individuals whose death was brought forward by just a few days or weeks, or correspond to those individuals whose health is at risk given that air pollution in China has already exceeded concentrations before the crisis. 5 In our opinion, improvements in air quality will result in a long-term benefit only if they are sustained in time.

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          Air pollution reduction and mortality benefit during the COVID-19 outbreak in China

          To control the coronavirus disease 2019 (COVID-19) outbreak, China adopted stringent traffic restrictions and self-quarantine measures, first in Wuhan and neighboring cities beginning Jan 23, 2020, and then 2 days later in all provinces in China (figure ). The countrywide ban on traffic mobility greatly reduced transportation emissions, whereas emissions from residential heating and industry remained steady or slightly declined. 1 In this Comment, we examine the change in air pollution and the potentially avoided cause-specific mortality during this large-scale quarantine. Figure Air pollution levels and avoided cause-specific deaths during the COVID-19 outbreak in China (A) The coronavirus disease 2019 (COVID-19) epidemic curve and quarantine timeline in Wuhan, Hubei province (outside of Wuhan), and China (outside of Hubei) from Jan 5 to March 14, 2020. New confirmed cases of COVID-19 were reported by the National Health Commission of China. (B) Daily nitrogen dioxide and PM2·5 concentrations from 20 days before the Chinese New Year and 50 days after the Chinese New Year in 2020 (blue line) and 2016–2019 (grey line). (C) The mean nitrogen dioxide tropospheric column density before and during the quarantine period in 2020. (D) The reduction in cause-specific deaths in China due to a decrease in air pollution during quarantine. As of March 14, 2020, new confirmed cases of COVID-19 in China reported by the National Health Commission decreased to 20 (four cases from Wuhan) (figure). By this time, most Chinese provinces had lowered the level of emergency responses. We thus defined the quarantine period as Feb 10 to March 14 and the period before quarantine as Jan 5 to Jan 20. Based on evidence from previous years, we excluded the Chinese New Year holidays to avoid reductions in air pollution that were unrelated to the quarantine (figure). We obtained daily concentrations of nitrogen dioxide (NO2) and PM2·5 in 367 Chinese cities from Jan 1, 2016, to March 14, 2020. We focused on NO2 and PM2·5 because both are traffic-related air pollutants whose emissions were substantially reduced as a result of the traffic bans and home quarantine, and both had well established concentration-response functions (CRFs) from one of the largest epidemiological studies in China on short-term mortality effects.2, 3 A difference-in-difference approach was then applied to quantify air pollution changes due to the quarantine. Specifically, we calculated changes in air quality during the quarantine versus before the quarantine period in 2020 and compared these findings with corresponding changes in the same lunar calendar periods from 2016 to 2019. This approach, which can also be interpreted as comparing changes in air quality in 2020 versus 2016–2019 during the quarantine period with those changes in the before quarantine period, also controlled for the long-term declining trend in air pollution because of China's clean air policy in the past few years. 4 To validate the air quality changes, we used satellite images from the Tropospheric Monitoring Instrument, which is onboard the Sentinel-5 Precursor satellite, to derive the mean NO2 tropospheric column density for periods during and before quarantine. We then calculated the avoided mortality attributable to decreases in daily NO2 and PM2·5 over China on the basis of short-term CRFs from a previous study of 272 Chinese cities (appendix p 2), and the cause-specific mortality data from the China Health and Family Planning Statistical Yearbook 2018.2, 3 In addition to total non-accidental and cardiovascular mortality, the cause-specific mortality for hypertensive disease, coronary heart disease, stroke, and chronic obstructive pulmonary disease (COPD) was also calculated. The attributable fraction (AF) method was used to estimate the daily avoided cause-specific mortality from the air pollution reduction. 5 AF is defined as follows: AF = 1 - e - β Δ c β is the cause-specific coefficient of the CRF and Δc is the air pollution changes due to the quarantine. AF is then multiplied by the daily cause-specific number of deaths and the total number of days during the quarantine period (34 days) to estimate the cause-specific avoided deaths. We found that, because of the quarantine, NO2 dropped by 22·8 μg/m3 in Wuhan and 12·9 μg/m3 in China. PM2·5 dropped by 1·4 μg/m3 in Wuhan but decreased by 18·9 μg/m3 across 367 cities (appendix p 3). The smaller reduction in PM2·5 in Wuhan is due to a similar decreasing trend in PM2·5 in 2016–2019. The pronounced decline in NO2 across China during the quarantine period was also detected by the Copernicus Sentinel-5P satellite with the NO2 tropospheric column density (figure). We estimate that improved air quality during the quarantine period avoided a total of 8911 NO2-related deaths (95% CI 6950–10 866), 65% of which were from cardiovascular diseases (hypertensive disease, coronary heart disease, and stroke) and COPD (figure, appendix p 4). Furthermore, we estimate that reduction in PM2·5 during the quarantine period avoided a total of 3214 PM2·5-related deaths (95% CI 2340–4087) in China, 73% of which were from cardiovascular diseases and COPD. Similar estimates were found with an alternative before quarantine period from Jan 1 to Jan 20 (appendix p 3–4). Our estimates should be interpreted with caution because of the potential overlap between PM2·5 and NO2-related mortality and the effect on mortality rate from disrupted health-care systems during the quarantine, which could have impacted the timely treatment of patients with chronic diseases. We used cause-specific CRFs from single-pollutant models because coefficients from two-pollutant models were not available. 2 Although there might have been some risk of double counting, results from published literature suggest that this risk is small because effect estimates for NO2 and PM2·5 were similar between single-pollutant and two-pollutant models.3, 6 Moreover, similar to previous epidemiological studies with outdoor air pollution, 7 exposure measurement error is inevitable since most people stayed indoors. Our estimates suggest that interventions to contain the COVID-19 outbreak led to improvements in air quality that brought health benefits in non-COVID-19 deaths, which could potentially have outnumbered the confirmed deaths attributable to COVID-19 in China (4633 deaths as of May 4, 2020). 8 Our findings show the substantial human health benefits related to cardiovascular disease morbidity and mortality that can be achieved when aggressive control measures for air pollution are taken to reduce emissions from vehicles, such as through climate mitigation-related traffic restrictions or efforts to accelerate the transition to electric vehicles.
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            Short term association between ozone and mortality: global two stage time series study in 406 locations in 20 countries

            Abstract Objective To assess short term mortality risks and excess mortality associated with exposure to ozone in several cities worldwide. Design Two stage time series analysis. Setting 406 cities in 20 countries, with overlapping periods between 1985 and 2015, collected from the database of Multi-City Multi-Country Collaborative Research Network. Population Deaths for all causes or for external causes only registered in each city within the study period. Main outcome measures Daily total mortality (all or non-external causes only). Results A total of 45 165 171 deaths were analysed in the 406 cities. On average, a 10 µg/m3 increase in ozone during the current and previous day was associated with an overall relative risk of mortality of 1.0018 (95% confidence interval 1.0012 to 1.0024). Some heterogeneity was found across countries, with estimates ranging from greater than 1.0020 in the United Kingdom, South Africa, Estonia, and Canada to less than 1.0008 in Mexico and Spain. Short term excess mortality in association with exposure to ozone higher than maximum background levels (70 µg/m3) was 0.26% (95% confidence interval 0.24% to 0.28%), corresponding to 8203 annual excess deaths (95% confidence interval 3525 to 12 840) across the 406 cities studied. The excess remained at 0.20% (0.18% to 0.22%) when restricting to days above the WHO guideline (100 µg/m3), corresponding to 6262 annual excess deaths (1413 to 11 065). Above more lenient thresholds for air quality standards in Europe, America, and China, excess mortality was 0.14%, 0.09%, and 0.05%, respectively. Conclusions Results suggest that ozone related mortality could be potentially reduced under stricter air quality standards. These findings have relevance for the implementation of efficient clean air interventions and mitigation strategies designed within national and international climate policies.
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              Meteorology-normalized impact of COVID-19 lockdown upon NO<sub>2</sub> pollution in Spain

              Abstract. The spread of the new coronavirus (COVID-19) forced the Spanish Government to implement extensive lockdown measures to reduce the number of hospital admissions, starting on March 14th 2020. Over the following days and weeks, strong reductions of nitrogen dioxide (NO2) pollution were reported in many regions of Spain. A substantial part of these reductions is obviously due to decreased local and regional anthropogenic emissions. Yet, the confounding effect of meteorological variability hinders a reliable quantification of the lockdown impact upon the observed pollution levels. Our study uses machine learning (ML) models fed by meteorological data along with other time features to estimate the business-as-usual NO2 mixing ratios that would have been observed in the absence of the lockdown. We then quantify the so-called meteorology-normalized NO2 reductions induced by the lockdown measures by comparing the business-as-usual with the actually observed NO2 mixing ratios. We applied this analysis for a selection of urban background and traffic stations covering the more than 50 Spanish provinces and islands. The ML predictive models were found to perform remarkably well in most locations. During the period of study, going from the enforcement of the state of alarm in Spain on March 14th to April 23rd, we found the lockdown measures to be responsible for a 50 % reduction of NO2 levels on average over all provinces and islands. The lockdown in Spain has gone through several phases with different levels of severity in the mobility restrictions. As expected the meteorology-normalized change of NO2 was found to be stronger during the phases II (the most stringent one) and III than during phase I. In the largest agglomerations where both urban background and traffic stations were available, a stronger meteorology-normalized NO2 change is highlighted at traffic stations compared to urban background ones. Our results are consistent with foreseen (although still uncertain) changes in anthropogenic emissions induced by the lockdown. We also show the importance of taking into account meteorological variability for accurately assessing the impact of the lockdown on NO2 levels, in particular at fine spatial and temporal scales. Meteorology-normalized estimates such as the ones presented here are crucial to reliably quantify the health implications of the lockdown due to reduced air pollution.
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                Author and article information

                Contributors
                Journal
                Lancet Planet Health
                Lancet Planet Health
                The Lancet. Planetary Health
                The Author(s). Published by Elsevier Ltd.
                2542-5196
                15 July 2020
                July 2020
                15 July 2020
                : 4
                : 7
                : e268
                Affiliations
                [a ]Centre for Demographic Studies, Autonomous University of Barcelona, Barcelona, Spain
                [b ]Climate and Health Program, Barcelona Institute for Global Health, Barcelona 08003, Spain
                [c ]Earth Sciences Department, Barcelona Supercomputing Center, Barcelona, Spain
                [d ]ICREA, Barcelona, Spain
                Article
                S2542-5196(20)30148-0
                10.1016/S2542-5196(20)30148-0
                7363419
                32681895
                7c3673b3-9dde-4686-8c4e-3a0600cd9ab0
                © 2020 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY-NC-ND 4.0 license

                Since January 2020 Elsevier has created a COVID-19 resource centre with free information in English and Mandarin on the novel coronavirus COVID-19. The COVID-19 resource centre is hosted on Elsevier Connect, the company's public news and information website. Elsevier hereby grants permission to make all its COVID-19-related research that is available on the COVID-19 resource centre - including this research content - immediately available in PubMed Central and other publicly funded repositories, such as the WHO COVID database with rights for unrestricted research re-use and analyses in any form or by any means with acknowledgement of the original source. These permissions are granted for free by Elsevier for as long as the COVID-19 resource centre remains active.

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