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Choosing a mask for fine dust

A higher rating is not automatically better. If it is hard to breathe you will take it off, and a mask around your chin filters nothing. Start by matching the rating to the day.

What the KF rating means

KF stands for Korea Filter. The number is the percentage of particles the mask filters out. Only masks certified by Korea's Ministry of Food and Drug Safety carry the mark.

Rating	Filtration	Good for
KF80	At least 80% of particles averaging 0.6 µm	Unhealthy days, ordinary trips out
KF94	At least 94% of particles averaging 0.4 µm	Very unhealthy days, Asian dust, long spells outdoors
KF99	At least 99% of particles averaging 0.4 µm	Specialised work. Overkill for daily use

If you are used to N95 or FFP2: KF94 is broadly comparable to those — the filtration requirements are close, though the test methods differ, so they are not officially interchangeable. KF80 filters noticeably less but breathes much more easily, which is why it is the everyday choice in Korea.

The higher the number, the harder it is to breathe. KF99 has considerable breathing resistance, which makes it impractical to wear for long and can be a real burden for older people or anyone with a respiratory condition. KF80 for ordinary days, KF94 when levels are high is a sound rule.

Surgical and cloth masks do not stop fine dust. Surgical masks are made to block droplets and have no particle-filtration standard; cloth masks let particles the size of PM2.5 pass straight through. If fine dust is the reason you are wearing one, look for the KF mark.

More important than the rating: the seal

Even a KF94 is undone by a gap between the mask and your face — air simply takes the easier route around the filter. The moment unfiltered air gets in, the rating stops meaning anything.

Press the nose clip to the shape of your nose. The bridge of the nose is where masks leak most.
Cover your chin completely. Worn under the chin, it may as well be off.
Breathe out hard. If you feel air escaping around your eyes or the sides, it is not sealed. Try a different size or shape.
Facial hair makes a seal difficult. On high-pollution days, trimming where the mask meets your face makes a real difference.

How long one lasts

Filtering masks are meant to be single-use. The filter catches particles electrostatically, and that charge weakens once it takes on moisture.

Replace it as soon as it is damp or dirty.
Never wash it or disinfect it with alcohol — that destroys the electrostatic filter and the filtration drops sharply.
If you take it off briefly, fold it or put it in a clean bag so the inside stays clean. Straight into a pocket is best avoided.
If the straps have stretched and the fit has gone loose, it is done.

When to wear one

On the Korean scale, Unhealthy (PM10 above 81, PM2.5 above 36) is where a mask starts to make sense. But the same number affects people differently.

With asthma, rhinitis or a heart condition, it is worth carrying one from the Moderate level.
Children and older adults are affected more than healthy adults.
Cycling or running increases how much air you move, so you inhale more particles at the same concentration. On those days, moving the workout indoors is a fair option.

Masks for children

Folding an adult mask down does not seal, so it achieves almost nothing. Use a small size made for a child's face. For infants, breathing resistance can be genuinely dangerous: the Korean Pediatric Society advises against masks for babies under 24 months. At that age, keeping the trip short or skipping it is the safer answer.