Intervention effect of low temperature plasma air purifier in highway toll booths

Background The serious air pollution of highway toll booths poses a high occupational exposure risk to toll collectors. It is urgent to develop purification methods suitable for airborne particles and microbial pathogens in highway toll booths.

Objective To verify the purification effect of low temperature plasma air purifiers on airborne particles and microbes in highway toll booths.

Methods Based on controlled-intervention design, we selected three toll booths in an expressway toll station as on-site experimental locations for 6 d (no-intervention period: the low-temperature plasma purifier was turned off in the first three days; intervention period: the purifier was turned on from 9:00 to 17:00 in the following three days). The indoor and outdoor PM_2.5 and PM₁₀ concentrations were continuously monitored during the study. At 9:00, 12:00, and 17:00 of every day during the experiment, indoor and outdoor air samples were collected to analyze the concentration of airborne culturable colonies with a plankton sampler. Airborne particle samples were collected in the outermost exit continuously from 9:00 to 17:00 every day during the experiment using a medium flow particulate sampler, and the species and relative abundance of fungi and bacteria contained in the samples were analyzed by gene sequencing. Independent-sample t test was used to compare the concentration of indoor PM_2.5, PM₁₀, and culturable colonies between the intervention period and the non-intervention period. α diversity analysis, β diversity analysis, and t test were used to compare the diversity and relative abundance of specific species of bacteria and fungi, as well as typical pathogenic bacteria and fungi in the samples between the non-intervention period and the intervention period to reflect the purification effect of low temperature plasma air purifier on airborne PM_2.5, PM₁₀, and microorganisms.

Results During the intervention period, the mean indoor concentrations of PM_2.5, PM₁₀, and culturable colonies were lower than those of the no-intervention period (P<0.01 or P<0.001). The ratios of indoor to outdoor concentration (I/O) of PM_2.5 and PM₁₀ during the intervention period were significantly lower than those of the no-intervention period (P<0.001), except the I/O of culturable colonies. Compared with the average concentration at 9:00, the average cleaning rates at 12:00 and 17:00 for PM_2.5 were 49.0% and 46.1%, for PM₁₀ were 49.7% and 45.4%, for airborne culturable colonies were 50.8% and 49.9%, respectively. The β diversity analysis showed that there were significant differences in composition at the level of species of bacteria, and at the levels of genus and species of fungi between the intervention and the no-intervention periods. The relative abundances of 10 species of bacteria such as Lactobacillus and 7 species of fungi such as Torula in the intervention period were significantly lower than those in the non-intervention period, but the relative abundances of fungi such as unclassified_f_cladosporiaceae, trichomerium, and cercospora were higher (P<0.05). For typical pathogenic bacteria, the relative abundances of Lactobacillus and Clostridium_sensu_stricto_1 during the intervention period were 73.5% and 86.9% lower than those in the no-intervention period, and the relative abundance of Talaromyces was 53.5% lower (P<0.05).

Conclusion Low temperature plasma air purifier has a good purification effect on indoor PM_2.5, PM₁₀, and culturable colonies in highway toll booths, and likely a limited effect on some fungi.