What was that smell in the air Thursday throughout the Philly region?

Did you smell that?

On Thursday, people in the Philadelphia region and South Jersey took to social media to discuss a peculiar odor in the air some described as the smell of burning plastic or chemicals.

Turns out, there was a meteorological reason for the stench, according to the Pennsylvania Department of Environmental Protection. It was not the result of a chemical spill, as some believed.

» READ MORE: Thousands of gallons of toxic phenol reportedly spilled this week at a Philly chemical plant

Virginia Nurk, a spokesperson for the DEP, explained that a condition known as an inversion occurred in the Philadelphia area. Residents in the city and across its suburbs noticed and experienced strong odors, mostly described as a burning or chemical smell.

An inversion, she said, “occurs when a layer of warm air is higher than a layer of cooler air, thereby trapping pollutants close to the ground.”

When that happens, pollutants from regulated industrial and commercial facilities, cars, and even wood burning fireplaces are unable to pass through the warm air layer and get trapped at a lower level — where people can smell them.

That condition remains until winds break up the atmospheric layers, releasing the pollutants, and allowing them to dissipate.

In fact, officials saw it coming. The DEP declared a Code Orange Air Quality Action Day on Wednesday in anticipation of the inversion.

Air Quality Action Days are meant to alert people with young children, the elderly, and those with respiratory problems, such as asthma, emphysema, and bronchitis, that they might be especially vulnerable to effects of air pollution and should limit outdoor activities.

Nurk said that even though officials believe the inversion was the cause, the DEP “will continue to investigate the area to ensure all regulated facilities were in compliance and not exacerbating conditions.”

However, as of Friday, no one source had been identified as the cause.