Overnight, Portland's air quality has gone from bad to worse—or in technical terms, from "unhealthy" to "very unhealthy."

Swiss Air quality monitoring company IQAir has given Portland an air quality rating of 204—the worst rating of any major city in the world—due to wildfires burning north and south of the city.

Other monitors have given Portland an even worse score on the Air Quality Index. According to the Environmental Protection Agency's monitor, some parts of Portland are pushing 300, or "hazardous," on the AQI scale.

AQI is determined by the ambient concentration of particulate matter in the air. Many smaller cities in Oregon are faring much worse—Salem's air quality is currently beyond what the scale can measure.

In Portland, without much wind or containment of the fires, it's unlikely that conditions will improve much over the next few days.

"What were seeing right now is this lull period," says Rebecca Muessle, a meteorologist at the National Weather Service's Portland office, "where winds are kind of calm and settled."

Over the weekend, southwest winds bringing air from the Pacific Ocean might clear out some of the smoke but not much.

"In general, it's going to stay pretty smoky through the weekend," says Muessle.

There might be some reprieve early next week: Though it's too early to tell for sure, some weather models have predicted precipitation on Monday for the Portland area.

For now, all residents—especially children, older adults and people with lung and heart conditions—are advised to avoid outdoor activities.