Carley Jones has seen the rent on her crumbling one-bedroom flat in Medway in Kent rise 40% in just three years, but she hasn’t got much for her money. A hole in her seven-year-old son’s bedroom wall lets in cold air so the heating is on non-stop. The plaster is coming away in the kitchen where the hot tap dribbles constantly. Jones sleeps in the living room, a broken smoke alarm hangs from the ceiling and laundry hangs, barely drying, in the damp bathroom.

“There are a lot of people going through this situation,” said the 23-year-old part-time school cleaner. “It is so easy for private landlords to get away with it because there are so many people desperate to find a property.”

Jones is not quite at the bottom of her area’s housing ladder. A few yards from her front door, a middle aged man has been living in the doorway of a vacant shop for six months – one of 4,750 people currently sleeping on Britain’s streets, according to the Ministry of Housing.

“I walk past him and think ‘what if that is me soon’,” she said. “I feel like a squatter in my own home. Rough sleeping is the next step down. There is a massive amount of stress and I now have depression and anxiety.”

The size of the private rented sector in England and Wales has doubled in the last 15 years. It now accounts for one in five households – more than the social rented sector. Several million people live happily in rented homes but a substantial minority do not. Some 756,000 households live in privately rented properties that are likely to cause residents to need medical attention, according to official figures. That’s well over 2 million people living in homes that actively damage their health.

On visits to several of the worst properties you can smell the problems before you see them. Damp and cold have a distinctive odour. In Newham, east London, a family of four from Bangladesh are paying £1,100 a month for a damp flat they believe has caused their nine-year-old’s asthma. Their three-year-old has a hacking cough that has ominously lingered for two months.

The rent is part-funded through housing benefit, because the father’s job as a mini-cab driver only earns him about £200 a week. Taxpayers are paying a private landlord for a family to live in a flat whose state of disrepair is jaw-dropping. The kitchen sink has collapsed through the rotten wooden work surface and cupboards are lined with black mould. In the living room water pours through the ceiling when it rains.

The collapsed kitchen sink and mould in the property rented by the Sultana family in Newham. Photograph: Jill Mead/The Guardian

“Morning and night, all I smell is damp,” said the mother, Jubly Rebeca Sultana, who said she has complained to her landlord for two-and-a-half years. The council issued an enforcement notice for repairs to be undertaken within two months, but with seven days to go until the deadline, nothing has been done.

Under the kitchen sink in the privately rented property of the Sultana family in Newham. Photograph: Jill Mead/The Guardian

A shortage of alternative cheap housing, councils that do not have the resources to tackle rogue landlords, and a failure among some property owners to behave ethically towards their tenants have all contributed to a problem which exacts a particularly heavy price on women and children.

“The private rented sector has always been in the worst condition,” said Peter Hobbs, head of the enforcement service overseeing more than 68,000 privately rented homes for Birmingham City Council. “An awful lot are in pre-1919 homes which are converted into houses of multiple occupation. Excessive cold is an issue and people struggle to afford heating.”

Jubly Rebeca Sultana and her two-year-old son. Photograph: Jill Mead/The Guardian

Almost half the households in Birmingham’s Ladywood area are privately rented. Since 2012, the council has made 30 successful prosecutions against landlords, with fines ranging from £400 to £22,000. It has identified the risk to tenants of “exploitation from irresponsible and in some cases criminal landlords and agents”. But cuts mean it is “struggling to meet the demand from tenants and to be able to target properties where the most vulnerable are likely to be housed,” it said.

A 36-year-old graphic designer who rents a home with four other professionals described a condemned gas boiler, condemned wiring (with exposed live wires behind the shower unit), no smoke detectors, damp walls, a leaking roof and ill-fitting window frames.

“The problem is systemic, until the government introduce a national house building programme and revise the right-to-buy scheme, the pressures will continue in the private sector,” she said.

Analysis by the Labour party of the English Housing Survey suggests that over 100,000 homes in the West Midlands feature category one hazards which pose a serious threat to the health or safety of people living there or visiting.

More than 100,000 housing association homes are believed to be in serious disrepair as well. Venetia Dolphy, 35, a hairdresser living in a Victorian housing association basement flat in southeast London, claims she has developed asthma and has had to stop work as a result of damp and mould. She said her children, aged seven and nine, suffer an allergic reaction to the mould which causes vomiting, diarrhoea and itchy, runny eyes.

Mould on the walls in the Sultana family’s flat in Newham. Photograph: Jill Mead/The Guardian

“The mould grew on my clothes, in the drawers and even the clothes in storage bags went mouldy,” she said. “It smelled and even when cleaned, the clothes made us itch and scratch. All the kids’ toys and school uniforms went mouldy.”

The property was supplied after she had previously been forced to leave another home because of damp and mould.

“It has been a living nightmare,” she said. “We moved and it started all over again.”

In south Gloucestershire, a 30-year-old mother of three young children has privately rented three homes in the last four years. Each was in severe disrepair, with damp and unsafe wiring, she said. For the last few months she has had no downstairs lighting, no cooking facilities, an unsafe chimney stack and a leaking roof.

When she complained, her landlord called her “a fantasist” and “over emotional”, she said.

“I suffer from mental health problems and this year has nearly destroyed me,” she said. “It has been like banging my head against a brick wall: self-doubt, a rollercoaster of emotions and I just want the best for my family. This isn’t good enough.”