Fig. 2From: Methodological limitations in studies assessing the effects of environmental and socioeconomic variables on the spread of COVID-19: a systematic review. Environmental data [81, 82]Smoothed curves for the relationships between daily temperature and daily levels of nitrogen dioxide and the number of daily cases of COVID-19. Spain, January 1, 2020 to April 14, 2021. The data were obtained from: [16]Back to article page