Subway air quality and its potential hazards have been concerned. However, compared with air pollution, the harm of subway air is often ignored by people.

The subway has become an important means of transportation for modern people and is closely connected with our daily life. During the morning and evening rush hours or around major holidays, subways are often faced with a comprehensive overload operation.

The subway is mainly built deep underground, connected to each other by a complex system of tunnels, forming a relatively enclosed space. Air exchange between the station and the outside world is mainly carried out through station entrances and exits and limited tunnel wind shafts.

In the ambient air quality standard, there is the saying of "four gases and two dust", of which four gases: carbon monoxide CO, nitrogen dioxide NO2, sulfur dioxide SO2, ozone O3, two dust: PM2.5, PM10. These pollutants are determined according to the overall requirements of the National Ambient Air Quality Standard (GB3095-2012) for six basic ambient air pollutant indicators.

The standard specifies an annual average concentration limit of 40μg/m³, a 24-hour average concentration limit of 80μg/m³, and a 1-hour average concentration limit of 200μg/m³. Why is it necessary to monitor nitrogen dioxide in the subway ambient air?

Basic item concentration limits for ambient air pollutants

The main reasons are as follows:

1) The potential hazards of nitrogen dioxide - Saint Kayan

According to experimental data, short-term exposure to nitrogen dioxide concentrations exceeding 200μg/m3 may have negative effects on animals and humans. Therefore, we must pay attention to the potential harm of this poisonous gas. For example, epidemiological studies have shown that increased bronchial symptoms in children with asthma are associated with the average annual concentration of NO2, and reduced levels of lung function development in children are also associated with current elevated concentrations of NO2 in outdoor air in urban communities in North America and Europe.

In the working environment, nitrogen dioxide may enter the human body through the respiratory tract, skin and digestive tract, becoming a poison to the nervous system. Long-term exposure to such an environment may have negative effects on human health, such as affecting enzyme activity, leading to lipoprotein metabolism disorders, and even cardiovascular disease. Inhaling high levels of NO2 (such as more than 50mg/m3) may trigger symptoms such as coughing, breathing difficulties and chest tightness, while in more severe cases, it may trigger pulmonary edema, causing damage to the respiratory tract.

2) The association of nitrogen dioxide with atmospheric pollution - Saint Keyan

In addition to its health effects, nitrogen dioxide is a gas that can have an impact on the environment. In ambient air, nitrogen dioxide can produce strong chemical reactions with many organic compounds, which is an important source of secondary pollution. For example, most of the NO2 in the environment is produced by the rapid oxidation of NO emitted by O3. Under sufficient light conditions, the reaction of nitrogen dioxide with volatile organic compounds (VOCs) will produce ozone and photochemical smog, which will not only exacerbate the PM2.5 pollution of the ambient air, but also affect atmospheric visibility.

3) The effect of nitrogen dioxide on atmospheric visibility - Saint Keyan

In addition, high concentrations of nitrogen dioxide may also lead to reduced atmospheric visibility. When the concentration of nitrogen dioxide in the air is too high, it produces a reddish-brown photochemical smog. This situation will not only interfere with people's vision, but also may affect traffic safety. Therefore, nitrogen dioxide is one of the important parameters of ambient air quality monitoring. As nitrogen dioxide is one of the main products of combustion sources, including urban thermal power generation, high-density traffic motor vehicle exhaust will produce NO2, China's NO2 concentration level is generally high. Therefore, we need to take effective measures to reduce nitrogen dioxide emissions in order to protect the environment and human health.