Particulate matter (PM2.5)

Particular matter refers to a complex mix of tiny solid and liquid particles in the air, such as dust, dirt, soot, and smoke. Since the chemical composition can vary, particulate matter is classified by size (PM2.5 consists of particles with a diameter of 2.5µm or less). These particles are so fine that they can get deeply embedded in the lungs and pass into the bloodstream. Together, many particles can be visualized as smoke or soot. Sources include vehicle exhaust, forest fires and coal-burning power plants.

      Nitrogen oxides (NOx)

Nitrogen oxides are part of the characteristic brown smog that can form a covering over cities. Nitrogen oxides can form when nitrogen and oxygen react at high temperatures. Naturally, this can occur in lightning storms; however, most nitrogen oxides form in vehicle engines and industrial furnaces. Nitrogen dioxide can dissolve in moisture in the lungs to form an acid, which can cause various types of respiratory and cardiovascular complications.