Penises are shrinking and genitals becoming malformed because of pollution, an environmental scientist has warned in a new book detailing the challenges facing human reproduction.

Dr Shanna Swan writes that humanity is facing an "existential crisis" in fertility rates as a result of phthalates, a chemical used when manufacturing plastics that impacts the hormone-producing endocrine system.

As a result of this pollution, a growing number of babies are being born with small penises, Dr Swan writes.

Her book, titled Count Down, examines "how our modern world is threatening sperm counts, altering male and female reproductive development, and imperilling the future of the human race".

Dr Swan's research began by examining phthalate syndrome, something observed in rats which found that when fetuses were exposed to the chemical they were likely to be born with shrunken genitals.

She discovered that male human babies who had been exposed to the phthalates in the womb had a shorter anogenital distance - something that correlated with penile volume.

The chemical has an industrial use in making plastics more flexible, but Dr Swan says it is being transmitted into toys and foods and subsequently harms human development.

Phathalates mimic the hormone oestrogen and thus disrupt the natural production of hormones in the human body, which researchers have linked to interference in sexual development in infants and behaviours in adults.

Dr Swan, who is a professor in environmental medicine and public health at the Mount Sinai Hospital in New York City, based her work on a series of peer-reviewed research studies.

One study published in 2017 found that sperm levels among men in Western countries had dropped by more than 50% over the past four decades after examining 185 studies involving close to 45,000 healthy men.

Dr Swan believes that the rapidly decreasing fertility rate means that most men will be unable to produce viable sperm by 2045.