Air filters are an essential part of ventilators that significantly reduce the risk of infection from airborne pathogens. As a result of treating sick patients, hospitals are at high risk of being contaminated by various viruses and bacteria, many of which become airborne. By acting as a physical barrier for particles above a certain size, particulate filters prevent these from entering a patient's respiratory system and causing additional complications to the patient's health.

Air filters are usually the final part of a ventilator before the mask.

Air filters are extremely simple: they consist on an inlet, an outlet and a porous membrane. Air is forced through the filter using positive pressure from the ventilator. As the air passes through the membrane, larger particles are trapped.

Membranes can be made from many porous or fibrous materials, such as paper, solid foam and cotton.

The effectiveness of a filter is characterised by a particulate size and a performance metric. The particulate size is the size of particle meant to be blocked by the filter. The performance metric is the percentage of that particle size that is blocked by the filter.

As a filter becomes finer, it restricts airflow more, so it is important to find a balance between the particle size and the static pressure drop across a filter.