Long-term air pollution levels modify the relationships between short-term exposure to meteorological factors, air pollution and the incidence of hand, foot and mouth disease in children: a DLNM-based multicity time series study in Sichuan Province, China

Abstract

Background: Epidemiological studies have investigated the short-term effects of meteorological factors and air pollution on the incidence of hand, foot, and mouth disease (HFMD). Several meteorological indicators, such as relative humidity and the diurnal temperature range (DTR), significantly modify the relationship between short-term exposure to temperature and HFMD incidence. However, it remains unclear whether (and how) long-term air pollution levels modify the short-term relationships of HFMD incidence with meteorological factors and air pollution.

Methods: We obtained daily data on meteorological factors, air pollutants, and HFMD counts in children from 21 prefecture-level cities in Sichuan Province in Southwest China from 2015 to 2017. First, we constructed a distributed lag nonlinear model (DLNM) at each prefecture-level site to evaluate the short-term impacts of meteorological variables and air pollutants on HFMD incidence. Then, we assessed the pooled effects of the exposures and incorporated long-term city-specific air pollutant indicators as meta-predictors to examine their potential modification effects by performing multivariate meta-regression models.

Results: We found that long-term SO₂ and CO concentrations significantly modified the short-term relationships between climatic variables and HFMD incidence. Specifically, high concentrations of CO (P = 0.027) and SO₂ (P = 0.039) reduced the risk of HFMD at low temperatures. The relationship between relative humidity and HFMD incidence was weakened at high SO₂ concentrations (P = 0.024), especially when the relative humidity was below the median level. When the minimum relative humidity (32%) was compared to the median relative humidity (77%), the risk ratio (RR) was 0.77 (95% CI: 0.51-1.17) in the 90^th percentile of SO₂ (19.6 μg/m³) and 0.41 (95% CI: 0.27-0.64) in the 10^th percentile of SO₂ (10.6 μg/m³).

Conclusion: Our results indicated that long-term SO₂ and CO levels modified the short-term associations between HFMD incidence in children and meteorological variables. These findings may inform health authorities to optimize targeted public health policies including reducing ambient air pollution and reinforcing self-protective actions to weaken the adverse health impacts of environmental factors on HFMD incidence.

Keywords: Air pollution; Environmental factors-HFMD association; Hand, foot, and mouth disease; Modification effect; Multicity analysis.

Fig. 1

Geographic regions, meteorological monitoring stations and air quality monitoring sites in Sichuan Province; the latter two sets of locations are presented on the division map

Fig. 2

Time series of HFMD cases, four climatic factors, and levels of five air pollutants in Sichuan Province between 2015 and 2017. We calculated the province averages by pooling daily data from 21 cities

Fig. 3

Overall relationship of HFMD risk with meteorological indicators and air pollution variables in Sichuan Province, with the median of each exposure serving as the reference value

Fig. 4

Estimated exposure–response relationships with the significant effect modifiers at relatively low and high levels (i.e., 10^th and 90^th percentiles)