Particulate is the term given to the minute particles of solid or semi solid material dispersed in the atmosphere. It is this dirt in the air, that is visible as a Brown Cloud, haze or smog. Particulates that range in size from less than 0.1 micrometer (µm) upto approximately 45 µm are designated as dust or Total Suspended Particulates. Particulates larger than that range tend to settle as dust and do not remain suspended, except during high winds. 

The human nostrils filter out 99% of the inhaled large and medium sized particles. The rest enter the windpipe where some of the inhalable particles cling to the protective mucous and are removed. Some of the smallest particles tend to get deposited in the alveoli (tiny air sacs in the lungs). The figure shows the likely regions of deposition of particulates in the respiratory tract.

These deposits of particulates slow down the exchange of oxygen with carbon dioxide in the blood, causing shortness of breath. The heart gets strained, because it works harder to compensate for oxygen loss. Respiratory diseases such as emphysema, bronchitis, asthma or heart problems get aggravated particularly in sensitive people such as the elderly and children. Inhaled particles may also be toxic (due to their chemical composition) and damage remote organs like the liver and kidneys. Swallowed mucous that is laden with hazardous particulate matter may damage the stomach. In addition particulates can also act as carriers of harmful bacteria, fungal spores and pollens.