The National Weather service has issued an air stagnation advisory for the greater Seattle area, meaning that "limited movement of the air mass over the advisory area will allow pollution to increase to dangerous levels." So yeah, try to avoid breathing when you step outside today.
Kidding aside, these sorts of things are not uncommon during winter. An advisory was also issued for the Portland area. People with severe allergies or respiratory illnesses are most affected, and the Weather Service advises those folks to consult their doctors if the air quality is bothering them.
A meteorological situation in which there is a major buildup of air pollution in the atmosphere. This usually occurs when the same air mass is parked over the same area for several days. During this time, the light winds cannot "cleanse" the buildup of smoke, dust, gases, and other industrial air pollution.
Air stagnation, as visualized by the New Hampshire Department of Enviornmental Services. Is that what mountains look like in New Hampshire?
The current advisory was issued yesterday at 3 p.m. and remains in effect until 4 p.m. today, though the actual atmospheric conditions will likely continue until midweek.
According to the official release from the Weather Service, the situation is the result of strong high pressure and warm air aloft that is trapping colder, denser air near the surface. This has produced "a prolonged period of poor atmospheric ventilation, which will allow pollutants like wood smoke and vehicle emissions to degrade air quality."