Table 1.

Descriptive statistics of the study population: distribution of the covariates and air pollutant concentrations among COVID-19 cases and among deceased subjects. Italy, 20 February 2020–15 June 2021 ($n=\mathrm{3,995,202}$ COVID-19 cases with nonmissing data).

	Cases [$n$ (%)]	Deaths [$n$ (%)]	Fatality rate (per 1,000)
Original population	4,038,677	125,238	31
Population (without missing)	3,995,202 (100.0)	124,346 (100.0)	31
Wave of the COVID-19 pandemic
1st: 20 February 2020–31 May 2020	201,210 (5.0)	35,440 (28.5)	176
2nd: 15 September 2020–15 December 2020	1,534,950 (38.4)	41,620 (33.5)	27
3rd: 16 December 2020–15 June 2021	2,259,042 (56.5)	47,286 (38.0)	21
Covariates
Age (y)
0–64	3,173,243 (79.4)	11,879 (9.6)	4
65–74	369,907 (9.3)	23,164 (18.6)	63
75–84	282,527 (7.1)	44,914 (36.1)	159
$85+$	169,525 (4.2)	44,389 (35.7)	262
Sex (females)	2,021,052 (50.6)	54,060 (43.5)	27
Clinical state at onset
Asymptomatic	1,740,258 (43.6)	23,252 (18.7)	13
Symptomatic	2,254,944 (56.4)	101,094 (81.3)	45
Socioeconomic deprivation index (quintiles)
Lowest	1,164,100 (29.1)	37,922 (30.5)	33
Low	784,627 (19.6)	25,420 (20.4)	32
Medium	790,333 (19.8)	24,023 (19.3)	30
High	718,853 (18.0)	22,016 (17.7)	31
Highest	537,289 (13.4)	14,965 (12.0)	28
Exposures [mean (IQR)]
${\mathrm{PM}}_{2.5}$ (${\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$)	17.1 (8.5)	17.8 (9.0)	—
${\mathrm{PM}}_{10}$ (${\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$)	25.6 (8.9)	26.1 (9.9)	—
${\mathrm{NO}}_2$ (${\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$)	23.1 (11.9)	23.6 (12.0)	—

Note: —, fatality rates for continuous covariates cannot be computed; IQR, interquartile range; ${\mathrm{NO}}_2$, nitrogen dioxide; PM, particulate matter; ${\mathrm{PM}}_{2.5}$, particulate matter with aerodynamic diameter $\le 2.5$ micrometers; ${\mathrm{PM}}_{10}$, particulate matter with aerodynamic diameter $\le 10$ micrometers.