Thick smoke from Canadian wildfires coated Chicago and the surrounding areas with haze as weather officials issued an air quality alert for parts of the Great Lakes, Lower Mississippi and Ohio valleys Tuesday morning.

According to the monitoring site IQAir, Chicago had the worst air quality out of 95 cities worldwide Tuesday.

Trent Ford, the Illinois state climatologist, said the worldwide index is essentially an average of the air quality monitors in a city. However, factors like time differences and the amount of monitors in a city make it difficult to use the index as a complete picture.

“But in general, to see cities like Chicago, which don’t, citywide, have this kind of perennial problem with air quality, at the top of that list, just gives you an idea of how unusual the conditions are right now,” Ford said.

As of 11 a.m., the air quality index had risen to a level considered “very unhealthy,” and it was continuing to rise in the afternoon, according to AirNow, a website that combines data from county, state and federal air quality agencies nationwide. This means everyone is at risk of experiencing health effects.