Pollen particles enter indoor areas through various routes, negatively affecting indoor air quality and human health. Therefore, it is important to measure the indoor and outdoor concentrations of pollen and to study the factors influencing the indoor/outdoor pollen concentration (I/O) ratio. Here, we review recent developments in pollen research, focusing on experimental studies on the I/O ratio and contributing factors, and suggest avenues for further research. Based on a literature search of 108 publications, the development, basic principles, advantages, and disadvantages of pollen samplers are summarized, and factors influencing the I/O ratio are summarized. The results showed that pollen type, ventilation, indoor ornamental plants, and human activities significantly affected the value of the I/O ratio, which mainly ranged from 0 to 0.5. Values of I/O ratio of 0.5-1 and higher were mainly found in studies with pollen diameters ranging from 15-30 mu m. A window that opened perpendicular to the dominant outdoor wind direction and a larger window opening area also increased the I/O ratio. Outdoor pollen can adhere to human surfaces and enter a room through human activity, increasing the I/O ratio. The results of this study will help other researchers to understand the state of research on pollen I/O ratios and hopefully provide suggestions for future research directions.
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