Dust masks are not "necessary" the same way most PPE is necessary. They are an option, possibly required or recommended or preferred by your work area or supervisor. Dust masks are only marginally to be regarded as PPE, and where they are "required," it is often due to a lack of understanding of the nature of their function. They can sometimes provide comfort against hot/cold air and nuisance (non-toxic) dusts, fumes, or mists, and in this they "protect" against discomfort. They are not respirators in the sense that this word is typically used, and they ARE NOT to be used for protection against airborne toxic particulate matter or for gases or vapors. If your work area or supervisor allows or requires the use of dust masks, study the information presented here and check the topic on the PPE training checklist in order to indicate that you have been informed of these facts.
It is sometimes necessary, or felt necessary by management, that workers be provided with dust masks for use in areas where nuisance (non-hazardous) dusts and other non-toxic airborne particulates are present, or to help buffer the effects of breathing particularly warm or cold air. The respiratory protection afforded by dust masks, if any exists, cannot be quantified (fit testing is impossible; that the filter material may have excellent properties is irrelevant; the face-mask boundary is where particulates will enter). They are never to be used as protection from illness or injury.
Most masks fit as pictured here; an elastic band above the ears and one below. Read the manufacturer's instructions.
Read the When and What sections above; they cover many of the limitations. Also be advised that the filter material will eventually become loaded with particulates and passage of filtered air will become impossible (meaning that ALL of the subsequent air you breath will be air pulled in from the imperfect seal between your face and the filter.
Some masks may be washed and re-used. Most are disposable or have disposable filters. Read the manufacturer's instructions.