Background As more residences are decorated, indoor formaldehyde pollution has drawn growing attention.

Objective This study investigates the formaldehyde pollution levels and change trends in newly decorated residences in Kunshan of Jiangsu Province.

Methods Forty newly decorated or redecorated residences in Kunshan were selected and investigated. The basic information of the residences were collected and the selected residences' indoor temperature, humidity, formaldehyde levels in winter (January to February) and summer (July to August) were evaluated in 2019. A total of 324 indoor air samples were collected from 162 sampling sites.

Results The average formaldehyde concentration was (0.021 9±0.011 9) mg·m^-3 and (0.096 3±0.107 2) mg·m^-3 in winter and summer respectively, with a significant difference (Z=-5.484, P < 0.001). The unqualified rate was 0.0% and 28.3% in winter and summer respectively, and the maximum value in summer was 1.040 3 mg·m^-3, 10.4 times higher than the relevant standard limit. The seasonal formaldehyde distributions were not different by newly-built or pre-owned residences, residence grade, bedroom number, decoration style, floor space, decoration costs, whether occupancy at detection, and heating mode (P>0.05). The formaldehyde concentrations in summer were different between different types of rooms (H=8.800, P=0.030) and whether most furniture is new (Z=9.259, P < 0.001). Formaldehyde concentration was positively correlated with temperature in winter and summer (winter:r=0.288, summer:r=0.151, P < 0.05), negatively with humidity in winter (r=-0.243, P < 0.05), but not with humidity in summer (P=0.841). The average days from complete decoration to formaldehyde detection was 130.1 and 303.3 d in winter and summer respectively. It was not found that the formaldehyde concentration decreased with the extension of days after completion of renovation in the two seasons (P>0.05).

Conclusion The concentration of formaldehyde in newly decorated dwellings is higher in summer than in winter in Kunshan. Formaldehyde concentration will increase obviously with the increase of temperature. New furniture is likely to contribute to formaldehyde pollution in Kunshan.