Ash and smoke are choking Seattle’s air for the second week in a row, as wildfires smolder in the Cascades and in British Columbia. As of Tuesday morning, the Air Quality Index in Seattle was at 181, a rating classified as “unhealthy.” In parts of the city, the index rose as high as 220, which is “very unhealthy.” … To put it in perspective, an AQI of 150 is roughly equal to smoking seven cigarettes in a day. That means residents should avoid being outside and exerting themselves, particularly people with heart and lung problems, the elderly, and children. The air quality in Seattle this week has been worse than in Beijing. …Though wildfires throw off particles of all shapes and sizes, the biggest health dangers come from the smallest ones, 2.5 microns or less in diameter. Known as PM2.5, these particles penetrate deep into the airways, causing inflammation, asthma attacks, and cancer.