The lingering stench is so nauseous that residents in North St Marys, Ropes Crossing and Tregear have to close their windows at least three times a week.

The odour is described as a chemical, acid-like smell which comes and goes throughout the year, depending on climate and air pressure, usually early morning and evenings.

A Ropes Crossing resident believes the source is Solveco, a waste treatment and recycling facility for hazardous, liquid, chemical and industrial wastes.

It aims for the best possible environmental outcomes before resorting to traditional disposal methods, according to its website.

The smell was so bad after a storm on November 20 that fire crews, including Hazmet, were called out to the Links Road site.

Dunheved fire station officer Garry Mitchell said the station had attended a number of calls in the last six months from businesses in Dunheved in relation to a gas smell but stressed none of the previous incidents involved Solveco.

The waste treatment facility declined to comment when asked for its views on the smell.

The resident rang the Office of Environment and Heritage last Monday, where he was told Solveco was being investigated.

"What are we breathing and is it harmful?" the resident asked.

A Penrith Council spokesman confirmed the Environment Protection Authority was investigating Solveco but understood there were no health concerns about the site.

The Office of Environment and Heritage was contacted last Wednesday and hadn't responded when the Star went to press yesterday.

Regardless who the culprit is, everyone just wants the smell to go away.

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