Seattle's was due to fires in surrounding regions. For whatever reason, smoke just accumulates in the Sound and lingers in the city, even for fires up in Canada. In LA, typically unless there is a fire in the immediate proximity, smoke doesn't really affect it that much. This article seems to be mostly about smog and ozone, rather than temporary air quality issues that don't really have a solution.

But yes, from what I've read, for a couple weeks last month, Seattle had the worst air quality in the world. And if you were here, you'd believe it.