Välisõhk

Välisõhk is the Estonian term for outdoor air, the portion of Earth's atmosphere that is outside buildings and urban canopies. It is the air people are exposed to when they are outside and serves as the baseline for environmental quality, weather, and climate processes. Välisõhk varies with location, weather, and human activity, and it is influenced by emissions from transport, industry, agriculture, and wildfires, as well as natural sources such as dust and pollen.

The composition of välisõhk is dominated by nitrogen (about 78%) and oxygen (about 21%), with small amounts of argon, neon, and other trace gases. Water vapor concentration fluctuates with humidity, and carbon dioxide is present at roughly 400 parts per million, a background level that can rise in polluted or urban areas. In addition to the basic gases, outdoor air contains particulate matter and various pollutants, including ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), carbon monoxide (CO), and volatile organic compounds (VOCs). These pollutants can be primary (emitted directly) or secondary (formed in the atmosphere).

Air quality is monitored by networks that measure concentrations of key pollutants and assign air quality indices. International and regional standards, such as the European Union’s Ambient Air Quality Directive and guidelines from the World Health Organization, set limits for pollutants like PM2.5, PM10, NO2, and O3 to protect health. Exposure to poor välisõhk is linked to respiratory and cardiovascular illnesses, with greater risk for children, the elderly, and people with preexisting conditions. Outdoor air quality is also affected by climate factors and urban form, and it interacts with indoor air through ventilation and infiltration.