Here is a brief explanation of fine dust.

Fine dust - also known as aerosols - is defined as airborne particles or droplets of solid or liquid materials. The solid particles may consist of inorganic and organic substances such as salts, metals, soot, greases and oils as well as biological material such as viruses, bacteria, spores, pollens and fungi. The liquid particles take the form of tiny droplets of fluid. Solid particles may be covered with a microscopic layer or film of liquid.

Key properties of these particles include their size and shape. The latter property determines how far these particles can penetrate the respiratory tract and which processes they initiate once inside.

In broad terms, we can state that,
fine dusts and ultra-fine particles act in accordance with an immensely simple and equally fiendish principle: the smaller the suspended particles are, the deeper they penetrate into our lungs, even entering our bloodstream, where they can wreak significant and harmful havoc. Thousands of people die every year as a result of inhaling fine dusts.

Even the slightest concentrations of fine dust should be considered as potentially carcinogenic.

(reference sources: Bundesverband Deutschland e.V. "Tod vom Allerfeinsten" [Death From The Ultra-Fine], ITG in BBU e. V., EPEA Internationale Umweltforschung GmbH, Arguk Umweltlabor GmbH, Literaturstudie zu Vorkommen und gesundheitlicher Bedeutung von Feinstaub in Innenräumen [Literature study on the incidence and health implications of fine dust inside rooms and buildings]