Table 4.

I/O (indoor/outdoor) ratio under different ventilation modes.

Ventilation Mode	Reference	Sampling Time Period	Building Type	Impact Factors of I/O Ratio	Results
Natural ventilation	[169]	1 December 2013–28 February 2014	Residential building	Outdoor PM2.5 mass concentration level	When the outdoor PM2.5 concentration is in the ranges of 0–33 μg·m−3, 34–65 μg·m−3, 66–129 μg·m−3, and ≥130 μg·m−3, the I/O ratios are 1.75, 1.05, 0.76 and 0.63, respectively
	[170]	April–December 2015 (one week per month, except in July and August)	School	Outdoor PM2.5 mass concentration level, ACH, wind speed and outdoor air temperature	The time average I/O is 0.69. It varies in the range of 0.1–5.46. The I/O ratio decreases with the increases of outdoor PM2.5 mass concentration level
	[171]	09:00–18:00, 13–15 March 2014	Laboratory complex building	ACH	48.7–57.3% of the PM2.5 pollutants come from indoor sources and the I/O ratio varies 0.90–1.23
Infiltration	[172]	September 2013–August 2014	Office	Outdoor dry bulb temperature, relative humidity ratio and wind speed	The average ACH is 0.10 under mild weather, 0.22 when the wind speed is 1.6–3.4 m/s, 0.39 when the wind speed is 5.5–8.0 m/s. The corresponding I/O ratios are 0.43, 0.56 and 0.62, respectively
	[82]	Winter, 2014	Residential building	Indoor pollution sources	When the indoor PM2.5 concentration reached its peak value, the I/O ratio was 0.67–0.89
	[76]	June 2013–August 2013; December 2013–February 2014	Office	Seasonal changes, wind speed and relative humidity ratio	The indoor and outdoor PM2.5 concentrations in winter were higher than those in summer and the corresponding I/O ratios were also higher in winter than in summer

ACH: air changes per hour.