Indoor particulate matter pollution and its influencing factors in a residential area of Lanzhou City

Background Indoor air pollution is closely related to human health, and particulate matter is one of the important indoor pollutants.

Objective This study investigates the indoor particulate matter pollution level and its influencing factors in residential buildings in an urban area of Lanzhou.

Methods In September (non-heating season) and November (heating season) of 2020, the air monitoring data in an urban area of Lanzhou City were collected. A primary school was randomly selected within 3 km of a monitoring station with the most serious air pollution. A total of 30 students in grade 2 in the school were selected through questionnaire survey, and their families were selected as monitoring sites. Indoor PM_2.5, PM₁₀, temperature, and relative humidity were detected in a living room and a bedroom in each of the 30 households. The residential characteristics were investigated by questionnaires. Chi-square test was used to analyze unqualified rates. Mann-Whitney U test was used for median comparison. Multiple logistic regression analysis was used to evaluate selected influencing factors of the levels of indoor particulate matter in different seasons. Spearman rank correlation was used to analyze the season-specific correlations of indoor temperature and relative humidity with particulate matter level, as well as the season-specific correlations between indoor and outdoor particulate matter levels.

Results The median concentrations of indoor PM_2.5 and PM₁₀ were 128.90 μg·m^-3 and 155.40 μg·m^-3, and the unqualified rates were 79.2% and 50.8%, respectively, in the selected 30 households. The median concentrations of indoor PM_2.5 and PM₁₀ were higher in heating season than those in non-heating season (Z=-6.716, -6.550, P < 0.05), and so were the unqualified rates (χ²=19.943, 29.037, P < 0.05). The median indoor temperature and relative humidity were 22℃ and 43%, and the unqualified rates were 5.8% and 12.5%, respectively. The unqualified rate of relative humidity was higher in heating season than in non-heating season (χ²=9.219, P < 0.05). In non-heating season, mopping or dust collection less than 1 time per day was a risk factor for unqualified PM₁₀, and the OR (95% CI) was 10.248 (1.747-60.107); opening windows less than 2 times per day was a risk factor for unqualified PM_2.5 and PM₁₀, and the ORs (95% CIs) were 5.196 (1.413- 19.110) and 12.079 (1.845-79.097), respectively. In non-heating season, temperature and relative humidity were negatively correlated with PM_2.5 (r=-0.771, -0.672, P < 0.05) and PM₁₀ (r=-0.664, -0.632, P < 0.05) concentrations. The median concentrations of outdoor PM_2.5 and PM₁₀ were higher in heating season (81.00 and 144.00 μg·m^-3) than in non-heating season (18.00 and 38.00 μg·m^-3) (Z=-9.141, -9.136, P < 0.05). In non-heating season, there was a weak positive correlation between indoor and outdoor PM₁₀ concentrations (r=0.294, P < 0.05). In heating season, the indoor and outdoor correlations of both PM_2.5 and PM₁₀ were significantly positive (r=0.597, 0.708, P < 0.05).

Conclusion There is indoor particulate matter pollution in selected households in the urban area of Lanzhou, especial in heating season. The levels of indoor particulate matter may be influenced by mopping and dust collection frequency, ventilation frequency, temperature, relative humidity, and outdoor particulate matter pollution.