Read Korea's Fine Dust Grade by the Number, Not the Word

Your Korean weather app shows 초미세먼지 (chomisemeonji, ultrafine dust) with the word 나쁨 (nappeum, "bad") next to it, and today was supposed to be the long walk up Namsan. Before you cancel anything, find the number behind that word. Korea grades PM2.5 on its own scale, and the same air can read "good" in a Korean app and "moderate" in an app built on the US index. The number tells you how far into 나쁨 you actually are, and the box you pick up at the convenience store should match it.

A Seoul boulevard with traffic and street trees under a thick gray haze that hides the top of a tall tower — A hazy fine-dust morning on a Seoul boulevard, May 2021. Photo: Hyeonyoung Lee 21, CC BY-SA 4.0, via Wikimedia Commons

The four words, as numbers

Korea's forecast grades use the 24-hour average of PM2.5 in micrograms per cubic meter (µg/m³). Here is the whole scale, so you can stop guessing what the word means:

Korean grade	PM2.5 (µg/m³)	Same air on the US AQI
좋음 (joeum, good)	0 to 15	Good up to 9, then Moderate (about 51 to 62)
보통 (botong, moderate)	16 to 35	Moderate (about 64 to 99)
나쁨 (nappeum, bad)	36 to 75	Unhealthy for Sensitive Groups up to 55, Unhealthy from 56
매우나쁨 (maeu nappeum, very bad)	76 and above	Unhealthy and higher

The right-hand column is a conversion: the Korean ranges run through the PM2.5 breakpoints the US EPA revised in 2024 (effective May 6, 2024), on the same 24-hour basis. Two things fall out of it. The top of Korea's 보통 (35) and the top of the US "Moderate" band (35.4) are almost the same line, so "moderate" means about the same thing in both systems. And 나쁨 is a wide bucket: 40 µg/m³ and 70 µg/m³ get the same Korean word, while the US scale splits them into orange and red.

That is why I would never plan around the word alone. A 나쁨 day at 38 and a 나쁨 day at 70 are different days. At 38, I would keep the outdoor plan but shorten it. At 70, I would flip the day: markets, museums and underground shopping arcades first, the hike only if the number drops.

One trap on the English side: the AirKorea English site leads with the CAI, a combined index for several pollutants, with Good at 0 to 50 and Moderate at 51 to 100. Those are index points, not µg/m³. If you see "45" there and "45" in a US app, they are not the same reading. Look for the PM2.5 figure itself before comparing.

What to do with a 나쁨 day

The official public guidance for poor air is plain: avoid long or strenuous outdoor activity, and stay out of it if your eyes sting or you start coughing or get a sore throat. For a visitor, the "long or strenuous" part is the one that bites. A two-hour palace-and-hill loop or a Han River bike ride is exactly that kind of activity, while a 15-minute walk between subway exits is not.

So the realistic move on a 나쁨 day is to reorder, not cancel. Put the walking-heavy stops on the day with the lower number. If you can only see one forecast, check the evening before. The National Institute of Environmental Research also runs a separate high-concentration flag two days out, which marks days when the daily PM2.5 average is expected to go over 50 µg/m³. It updates four times a day, at 5:00, 11:00, 17:00 and 23:00, and has covered the Seoul metropolitan area since November 28, 2022. If that flag is up for your hiking day, swap days while you still have the choice.

Watch for two stronger words too. A 주의보 (juuibo, advisory) for PM2.5 goes out when the hourly level stays at 75 µg/m³ or more for two hours, and a 경보 (gyeongbo, warning) at 150 or more for two hours. Those are the days to keep outdoor time short no matter what the itinerary says.

If you have asthma, a heart or lung condition, or you are traveling with small children or older parents, the guidance is stricter than the general-public line above. Talk that through with your own doctor before the trip rather than deciding at the hotel.

The mask aisle: two words on the box

Masks for dust are sold at pharmacies (약국, yakguk), supermarkets and convenience stores (편의점, pyeonuijeom). The box design will not help you, but two printed words will.

First, 의약외품 (uiyagoepum, quasi-drug). That marking means the product is licensed by the Ministry of Food and Drug Safety as a 보건용 마스크 (bogeonyong maseukeu, health mask). A box that says it "blocks fine dust" without this word is not one of the licensed products.

Front of a Korean mask box showing KF94 in large letters and a round label reading 의약외품 식약처허가 — The KF grade and the 의약외품 mark sit on the front of the box. Photo: Fallaner, CC BY-SA 4.0, via Wikimedia Commons (cropped)

Second, the KF grade. KF80 is rated to block at least 80% of particles averaging 0.6 µm, and KF94 at least 94% of particles averaging 0.4 µm. The trade-off is that a higher number blocks more but is harder to breathe through, so KF80 is a sensible pick for a long walking day at the low end of 나쁨, and KF94 for the high end or an advisory day. You may also see 황사 (hwangsa, yellow dust) on the box. That is the spring dust from the continent, and the same KF-rated masks are meant for it.

Here is the mix-up to avoid. KF-AD is not a stronger KF94. The "AD" box is the 비말차단용 (bimal chadanyong, droplet-blocking) type that appeared in 2020, and the ministry ranks it below KF80 for blocking fine particles. It is easier to breathe through, which is the point of it, not extra protection. On a high 나쁨 day, put it back and take the KF80 or KF94.

The limit of all this is fit. A KF rating assumes the mask seals over the bridge of the nose and around the whole face, so a KF94 worn with gaps at the cheeks is not doing KF94 work. Skip the trick of tucking a tissue or towel underneath, since it breaks the seal, and treat each mask as single-use rather than washing it. If a product name looks unfamiliar, search it on the drug database at nedrug.mfds.go.kr before you pay.

Tonight, open the forecast, find tomorrow's PM2.5 number, and decide which day gets the long walk before you buy anything.

Sources checked November 7, 2026

All About K-Everyday

Search This Blog