Southall and Hayes residents were left choking after enduring constant "suffocating" smells during the hot weather.
The stench, described as smelling like "burning plastic or tar" has left residents with headaches and suffering through the heatwave as they couldn't open windows.
Aastha Narang, 27, has had to put up with the odour for weeks at her home, her work, and her young son's primary school - all in Southall.
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She said: "It's a stench of burning plastic or tar, coming from the blue building, the gas tower near Hambrough Primary school.
"When I have gone past it's that asphalt smell, it's very strong, I can smell it all the time in my flat.
"Ealing Council say they cannot identify where the smell is coming from.
"In the heat it's worse as we cannot open windows - the fact is I had a sore throat, I was coughing really badly.
"I work in a nursing home and some of the residents have asthma so I can't imagine how bad it is for them.
"The stench is so strong and I have to smell it at work, at my son's school and at home. It gives me headaches.
"If we have another heatwave it's just going to be unbearable, last week my son even said to be he can smell it can you close the window."
But Tony James, chairman of the Signal Building Residents Association at High Point Village in Hayes, believes the stench is coming from the FM Conway asphalt plant.
He said: "Half of the apartments at Signal Building face east and are in direct line of the site with no other buildings or barriers in the way.
"Last night and this morning we were hit with a constant odour from 7pm lasting until at least 7am and this is indicative of our experience over the last seven to 10 days.
"The apartments at Signal Building suffer disproportionately with very high temperatures during warm spells.
"My own flat peaked at 36.5 degrees last week, so having windows and doors open is essential to trying to keep cool yet our evenings are filled with this foul smell.
"We raised the same problems with Hillingdon Council last summer but it seems any moderate rise in temperatures leaves us suffering again."
MP calls for action from local authorities
Hayes and Harlington MP John McDonnell previously campaigned to have the Conway factory shut down unless its reduces pollution levels.
He told getwestlondon: "Although there has been some improvement after my earlier campaign this plant continues to cause problems from time to time.
"The prevailing wind makes it worse for many Southall residents.
"The two councils should get together to sort thus issue out once and for all. I continue to be worried about any long term implications for the health of local residents."
Ealing Council says it has received a large number of odour complaints in the last three weeks, some "from the vicinity of industrial tarmac and roadstone coating plant centres in Hayes".
A spokesman for the council said: "Although they lie within Hillingdon Council’s jurisdiction, we have responded to these complaints and are working closely with Hillingdon and the business concerned to find the source of the smell and put a permanent solution in place."
“In addition, some odours had come from a cleaning operation of one of the tanks on the Southall Gasworks site," he said.
"Following an on-site meeting last week, the treatment operation has been stopped until additional odour suppressing equipment has been installed.
"We will continue to pro-actively monitor the site on a regular basis.”
Digging odorous contaminated soils 'aggravated' the smell
Hillingdon Council said both the recent heatwave and the cleaning up of the decommissioned Southall gasworks site in Ealing have been the cause of odour complaints.
A spokesman said: "The digging out of odorous contaminated soils at this site has aggravated the smell from the tarmac-producing factories nearby.
"Hillingdon Council officers met FM Conway's senior team on Friday to continue to press for better controls to be put in place under their environmental permit."
Odour control system in place
FM Conway say they take the health and safety of locals "seriously" and have invested in new equipment.
A spokeswoman for the plant said: "Within the last two years this has included installing a state-of-the-art environmental and odour control system which captures particulates and smells inside the plant itself.
"These systems are designed to operate in a range of weather conditions, including during unseasonably hot periods as we experienced in June.
"As a result, our emissions continue to sit well below regulatory requirements. We are in regular contact with the environmental health and permitting teams from the London Boroughs of Hillingdon and Ealing.
"Representatives from both councils toured our facility last week and were satisfied that it continues to operate within required limits as well as to the high standard that we set.
"Within the Southall Green area there are a number of other industrial and development sites.
"As a major presence in the local community we have a responsibility to share our expertise and practices, and are in dialogue with both Hillingdon and Ealing Councils and the Environment Agency to do so."
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