There are several different types of dust mask available and you need to make sure you have the right one for the job. For basic protection disposable dust masks are available which simply cover the mouth and nose .This type of mask is suitable for dust but offers no protection from toxic or harmful substances and is therefore not suitable for work in environments with even very low levels of these substances.
For more advanced protection a respirator style mask is available which includes a filter purifying the air which is being breathed in. The strength and number of filters determines what level of protection the mask provides The level of protection offered by the masks Is described in terms of how many times higher than the Workplace Exposure Limit (WEL) the mask can filter. Therefore the higher the number the WEL is the greater the level of protection. Some valve masks are only designed to protect against things such as aerosols therefore the protection level of the mask should always be checked for suitability.