Toxic substances released while a 150-year-old gas works is being redeveloped have reached levels that "present a threat to health", an expert has said.
Residents living near the site in Southall, west London, have complained of poor health and breathing difficulties since work began in 2016.
Tonnes of earth have been cleaned during the work and the developers said they had met "regulatory requirements".
Public Health England said there was "minimal" risk to health at the site.
Berkeley Group is regenerating the 88 acre (36 hectare) former industrial site at Southall Waterside to create thousands of homes, shops, a primary school and leisure facilities.
The Marsden family are among the residents who have said they noticed a terrible smell, described as a mix of chemicals and petrol, when earthworks first began and they had all suffered ill health since.
David Marsden, 52, said he had lost count of the number of chest infections he had suffered, while his five-year-old son Zion was hospitalised with viral-induced wheeze.
"He's had to take a brown steroid inhaler morning and night basically just to keep his airways open. As parents it's incredibly frightening to see your child like that and you feel powerless and helpless," he said.
Ahmed Rahman, whose elderly parents lived in a house by the construction site, said his 82-year-old father Khawaja complained about "finding it difficult to breathe" after works began.
"He went into hospital [in February 2018] and then he got pneumonia. Within three weeks he passed away," he said.
A risk assessment undertaken by Public Health England into substance levels at the site said they had "remained below levels likely to cause acute or short term health effects".
However, the BBC showed reports carried out for Berkeley Group in 2018 to environmental health and air quality scientist Prof Roy Harrison who said levels of some substances "do present a threat to health".
"For some individuals one might expect some exacerbation of asthma symptoms and in the long term it's possible that there may be an increase in cancers," he said.
Prof Harrison said the fact the area still smelled suggested "some contamination remains on the site and there is a potential for future risk".
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In a statement, Berkeley Group said the cleaning of all soil "was undertaken in accordance with all regulatory requirements and the work has been closely monitored by the London Borough of Ealing and the Environment Agency".
Ealing Council said it had "received continuous assurances from the Environment Agency that proper mitigations are in place at the site".
"We will continue to monitor the site and want to hear from anyone experiencing problems," it said.