Of the 9 million deaths due to pollution in 2019, air pollution (including indoor and the environment, still totaled 6.67 million worldwide, about three times the amount of water and soil pollution combined. The main causes of death caused by air pollution are: cardiovascular diseases, respiratory diseases, malignant tumors, etc.

Scientific research points out that there is no clear threshold for the impact of some air pollutants on human health, and that human exposure to low concentrations of pollutants still has health risks. Take PM_2.5 as an example, when the human body is long exposed to low concentration of PM_2.5 pollution, there is still a risk of lower respiratory tract infection, stroke and other diseases. The risk of death per unit of PM_2.5 concentration in the low PM_2.5 concentration range will be more significant with the decrease of pollutant concentration.

In June 2021, a city center for Disease control and Prevention. A total of 19 indoor or outdoor monitoring points were deployed to study the differences and changes of the indoor and outdoor concentrations of various typical pollutants through data collection, visual treatment and statistical analysis, and to provide basic data support for the evaluation of their health effects.

Taking the sampling data of outdoor locations in a hotel room and bus station as an example, the proportion of indoor and outdoor fine particles is obviously different; indoor fine particles mainly increase the proportion of outdoor coarse particles, which may be related to the better penetration of doors and Windows and the higher indoor resuspension efficiency.

The monitoring results also showed that the indoor concentration of NO₂ and O₃ in the environment without indoor combustion or cooking sources was lower than the outdoor concentration of the same period, which may be related to the heterogeneous chemical reaction of these two types of highly reactive oxidizing gases on the surface of indoor building materials (O₃ may also have gas phase reaction with active VOCs in the room). The study shows that even if the indoor O₃ concentration is far lower than the current indoor air quality standards in China, its short-term adverse effects on children's cardiopulmonary health can still be observed, and most ozone exposure occurs in the indoor environment, so the O₃ in the indoor environment has an important impact on human health. At some of the points, such as the bus station shown in the figure, the NO₂ concentration of outdoor stations is higher than the standard station, which is likely to be related to the local automobile exhaust emissions.

The CO sensor based on the principle of electrochemical reaction is dependent on the relative CO concentration of the CO concentration monitoring results: the CO concentration is generally significantly higher than VOCs, so the sensor signal is basically considered the CO from emission sources such as traffic, catering; but in indoor environment, the concentration of VOCs may be significantly higher than that of CO, so the monitoring signal of the sensor can be regarded as the characterization of indoor VOCs concentration to some extent. The high "CO" signal measured by electrochemical sensors in a hotel room in a city indicates that the VOCs concentration in the room is significantly higher than that in the outdoor environment, and may come from volatilization or emission from furniture, walls or other indoor sources.