What are fine black particles, chiefly composed of carbon, produced by incomplete combustion of coal, oil, wood, or other fuels called?
Soot
Soot includes the fine black particles, chiefly composed of carbon, produced by incomplete combustion of coal, oil, wood, or other fuels. The common trait of soot particles is that they are extremely tiny – 2.5 micrometers or smaller in diameter.
Because if it's very small size, it can travel deep into the lung, and can do some serious damage.