From burning plastic to wood and even chlorine. Folks in the region have been so concerned with unusual odors in the eastern portion of Montgomery County, that they've been calling 911.
But, thankfully the issue is not an emergency.
When you step outside, you see the fog and may wonder, "What's that smell?"
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While maybe you didn't notice it, Dr. Stephen Strader of Villanova University says that he did.
"A box truck drove by and I smelled the exhaust and then it just never went away," he told NBC10.
And, unlike so many who went to social media to ask what it was, some told others where it was and what it smelled like.
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Complaints were posted from East Norriton to Gywnedd with people saying it was chlorine or burning wires.
Dr. Strader is an associate professor of geology and the environment. He said he agrees with the Pennsylvania Department of Environment Protection who said we're experiencing an atmospheric inversion.
An atmospheric inversion is when warm air is held down and those environmental impacts - like exhaust, industrial aromas and trash -- all get stuck in the air and linger longer.
"That suppresses all that air from rising up. So, along with that heavy, dense fog anything that's pollution or wildfires or just burning or anything that normally would get mixed into the atmosphere is kind of held down. Almost like the lid on a soda bottle," Dr. Strader explained.
Once the weather patterns change, air quality is expected to improve with a change in the winds.
Dr. Strader says that we are lucky because other parts of the world put up with these types of atmospheric inversions nearly every day.
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