A massive mulch fire at a factory storage yard on Detroit's west side is pushing smoke across the city and over the border into the Windsor area.

That's what's behind the burning smell in the air Wednesday morning.

Detroit Fire Department Chief James C. Harris said it broke out at Detroit Mulch on Prairie Street around 6:30 p.m. Tuesday, and it's expected to burn for a while.

"I'd say it's the size of six football fields," Harris told CBC News.

It's unclear at this point how the fire started.

WATCH | Click on Detroit video shows fire crews battle massive mulch fire: 

[embed]https://www.youtube.com/watch?v=Y1Lw3AeUyac[/embed]

He's encouraging people in Detroit and Windsor to shut their windows.

"The wind could change direction at any point."

Harris said they've been rotating through approximately 15 crews and fighting the fire for more than 12 consecutive hours, as of 8 a.m. Wednesday.

He said some of the mulch piles are "substantial" and high, making it difficult for crews to put out. 

WATCH | WXYZ-TV Wednesday a-m video of massive mulch fire in Detroit: 

[embed]https://www.youtube.com/watch?v=nyXmJCItICQ[/embed]

The Windsor-Essex County Health Unit (WECHU) says air pollution, regardless of its source, poses significant health risks.

In a statement, the health unit says it's monitoring the fire situation, and as of 11 a.m. Windsor's Air Quality Health Index (AQHI) was at a level of five, indicating a moderate health risk.

Dr. Mehdi Aloosh, the local medical officer of health, says at-risk groups, such as seniors and people with underlying diseases such as lung and heart conditions, should consider reducing or rescheduling strenuous activities outdoors if they are experiencing symptoms.