The Washington DC region experienced a dramatic decline in air quality last week after smoke from Canadian wildfires spread southward, blanketing the District in a dense haze.

The smoke pushed DC into Code Red alert conditions before pollution levels worsened to the rarer Code Purple category later in the week.

More: How bad is the air pollution in Washington DC today? See the photos

Forecasters described the smoke event as one of the most significant to impact the DC region in years.

Just how historic was it, and is more smoke on the way? Here's what we know.

Wildfire smoke from Canada and Minnesota is causing poor air quality across the northern U.S. See the impact of wildfire and smoke across North America. Smoke rises from the Lussier River Wildfire (N20809), north of Cranbrook, British Columbia, Canada July 11, 2026 in an aerial photograph.

A helicopter drops water on the Brunswick Complex wildfires near North Bend, British Columbia, Canada July 11, 2026. Air quality alerts have been issued for sensitive groups, including children, older adults, and those with respiratory issues.

Wildfires smoke caused a hazy sky in Worcester, Mass., July 15. Particulate matter, a type of air pollutant made of small particles of solids or liquids that are suspended in the air, is one of the main components of wildfire smoke, according to the National Weather Service. The particles are smaller than a strand of human hair and can enter one's respiratory system, leading to a slew of symptoms.

Wildfire smoke from Canada and Minnesota is causing poor air quality across the northern U.S. See the impact of wildfire and smoke across North America. Smoke rises from the Lussier River Wildfire (N20809), north of Cranbrook, British Columbia, Canada July 11, 2026 in an aerial photograph.

Capital Weather said Reagan National Airport recorded 42 consecutive hours from Thursday to Saturday with smoke in its weather observations. That was the most — by 11 hours — since November 1952.