Known by foreigners as "Yellow Dust" it's basically particles in the air that are big enough to be inhaled into the lungs. Sometimes you can even taste and feel it n your nose and throat. If you're travelling to Korea, Yellow Dust is something you should know about beforehand so you can prepare.
The particles originate in the dry regions of China and Mongolia, and blow onto Korea. I know it sounds crazy, but it's been happening for decades. Deforestation, rapid industrialisation and pollution are to blame.
Typically Yellow Dust happens in the Spring time (March - May), but it can happen any time. In sufficient concentration, the fine particles can obscure visibility, irritate soft tissues in the eyes, nose, mouth and throat. Because of the high concentration of minerals and other pollutants, it can cause or exacerbate respiratory and cardiovascular problems. Pinkeye is a common side effect in both adults and children. The dust can also damage sensitive equipment, such as computers, etc.
This website is a great tool. It has English warnings for Yellow Dust and a meter on right of the page, so you can track the Yellow Dust levels day-to-day. Check it out here.
I have terrible asthma and lung issues, so the Yellow Dust really affected me. For weeks at a time I couldn't leave the house without a mask, and even so it made me sick. You feel like a bit of a fool wearing the mask, but it's worth it. It's also pretty common to even see locals with a mask or scarf around their mouths during yellow dust time. Every corner store / 7 Eleven sells masks for less than $5, usually they are around $2. I got a cute Hello Kitty mask because it seemed like the right thing to do.