Having been in the USA since January 31st 2014, I missed the most recent episode of this, which was big, sustained. I saw my friend Jared mention it. The "Air-Korea" website confirms it. Pollution data easily accessible there.
The pollution "spike" you see there (yellow and orange) lasted eight full days, from late evening February 21st all the way through till late evening March 1st. (A second [30-hour] spike occurred two days later.)
It's in the 60s Fahrenheit (15-20 C) here in Arlington, Va., as I write, after snow last week. I'm pretty sure that most places in the USA (including Arlington) would have blue boxes on the above chart most every day.