The Chicago area's "milky white" sky is caused by smoke from Canadian wildfires blowing over the upper Midwest, which could contribute to worsening air quality in the region today.

The smoke originated from fires in western Ontario and eastern Manitoba, making the sky hazy in spite of clear weather, the National Weather Service in Chicago reported on Twitter Monday.

"The sky may have a milky white look to it, especially over parts of far northern IL," the Twitter post said.

"Could see some of this reach pretty low and maybe even impact air quality some," the weather service said in an earlier Twitter post.

The Environmental Protection Agency reported moderate air quality in most of the Chicago area on Monday, which means pollutants could cause a health concern for a very small number of unusually sensitive people.

But today, much of the Chicago area is expected to see air quality listed as "unhealthy for sensitive groups," which means older people, children and people with lung diseases.