How Masks and Respirators Work

Why does a respirator stop particles that a loose cloth mask lets through, and why can both fail when they gape at the cheeks? Every mask does two jobs: it filters the air that passes through its material, and it either seals to the face or does not. This sheet explains both jobs, then sets out the plain meaning of droplets versus aerosols and of protecting yourself versus protecting others. Masks reduce exposure. They do not guarantee protection.

How a filter catches particles

The NIOSH science bulletin on N95 respirators and surgical masks (posted 14 October 2009) says the filters in modern surgical masks and respirators are fibrous: flat, nonwoven mats of fine fibres. Fibre diameter, porosity and thickness all affect how well they collect particles. In every fibrous filter, three mechanical mechanisms operate.

Inertial impaction: larger or heavier particles have too much inertia to follow the air as it bends around a fibre, so they strike it.
Interception: particles that pass close to a fibre touch it and stick.
Diffusion: very small particles are knocked about by air molecules, drift off the airstream and meet a fibre.

NIOSH assigns impaction and interception mainly to larger particles and diffusion mainly to smaller ones. Some filters also use charged fibres. Then a fourth mechanism, electrostatic attraction, pulls oppositely charged particles to the fibre, and NIOSH says it does not favour any particle size. It adds that this matters for N95 filters because it improves collection without making the filter harder to breathe through.

Why a filter is not a simple sieve

Many people picture a net with holes too small for particles to pass. NIOSH calls this the most misunderstood aspect of filter performance and states that filters do not act as sieves. There is a particle size, the "most penetrating particle size", at which none of the three mechanical mechanisms works especially well. Filters are best tested at that size, because particles both smaller and larger are then caught even better.

NIOSH's certification test uses a salt aerosol of about 0.3 micrometres at an airflow of 85 litres per minute. A 2021 peer-reviewed review notes that more recent studies put the most penetrating size nearer 0.04 to 0.06 micrometres. A filtration percentage therefore only means something alongside the test that produced it, which the standards behind masks explains.

The leak path: fit matters as much as the filter

Air takes the easiest route. NIOSH's public guidance (last updated 25 March 2025) says a respirator or mask can only filter particles that try to pass through its filter material. If gaps exist between face and mask, small particles you cannot see can enter through them. The bridge of the nose and the sides are common leak points, and facial hair is a common cause. Tight-fitting respirators are designed to seal, so breath is forced through the filter. Masks are designed to cover the nose and mouth without sealing.

The 2009 bulletin gives a sense of scale. In a laboratory study of five surgical masks with "good" filters, 80 to 100 percent of subjects failed a qualitative fit test, and fit factors ran from 4 to 8, which NIOSH equates to 12 to 25 percent leakage. A fit factor is the particle concentration outside the mask divided by the concentration inside. By contrast, NIOSH says even the least protective respirator type, a negative pressure half mask, must reach a fit factor of at least 100 (1 percent leakage). The reverse holds too: a mask that fits well but has a poor filter cannot give high protection. See fit and seal, and masks with glasses and hearing aids for a common cause of gaps.

Droplets and aerosols

The two words describe size. In its scientific brief of 9 July 2020, the World Health Organization wrote that respiratory droplets are larger than 5 to 10 micrometres across, while those smaller than 5 micrometres are called droplet nuclei or aerosols. It defined airborne transmission as spread through droplet nuclei (aerosols) that remain infectious while suspended in air over long distances and time. WHO now marks that brief as outdated for the latest transmission information, so read the sizes as WHO's wording on that date.

The distinction matters because filter performance follows particle size. NIOSH notes that a filter's collection efficiency depends on particle size, not on whether the particles are bioaerosols or inert dust.

Source control and wearer protection

Masks work in two directions. Wearer protection means filtering what you breathe in. Source control means reducing what the wearer breathes, coughs or speaks out into the room. The US Food and Drug Administration (page modified 21 October 2024) describes face masks as intended for source control, and a surgical mask as a loose-fitting, disposable barrier between the wearer's mouth and nose and contaminants in the immediate environment.

NIOSH says masks and respirators may filter and block particles you exhale, but that no standard test measures the protection a respirator or mask gives to others. A loose fit lets particles and droplets escape through the gaps, and a respirator with an exhalation valve may protect others less well because some air leaves through the valve. NIOSH also ranks respirators as more protective than masks, with disposable and cloth masks the least effective at reducing exposure. See types of masks and respirators for how the groups differ.

This is general information. NIOSH advises anyone with a heart or lung condition to ask a doctor before using a respirator or mask, and to move to cleaner air and remove it if breathing becomes difficult or dizziness starts. Your own clinician has the final word.

The short version

A filter catches particles through impaction, interception, diffusion and, in some materials, electrostatic charge, rather than by acting as a sieve. A good filter does little if air leaks around it, so seal and filter both count. Droplets and aerosols are a size convention that helps explain why loose masks block some things and tight respirators block more.