Dr. Shanna Swan

Dr. Shanna Swan opens by restating the thesis of her book Countdown: the modern world is threatening sperm counts, altering male and female reproductive development and, in her framing, imperiling the future of the human race. She attributes the decline in reproductive health largely to chemicals people are surrounded by every day, though she stops short of calling them the sole cause.

playing a big part in it not the only part but a big part in it

The mechanism she describes is hormonal interference. These chemicals, grouped together as endocrine disruptors, can raise levels of some hormones, lower others such as testosterone, and affect thyroid hormone. They can also change how hormones are transported and made available in the body. Swan argues that the consequences matter more than the mechanism sounds: by her account, sperm counts, miscarriage rates, fertility rates and testosterone levels are all worsening at about one percent per year.

Swan has focused on one class of endocrine disruptors, phthalates, because they can lower testosterone, which she calls critical to both men's and women's reproductive health. She says they are very common, and that a urine sample sent to the Centers for Disease Control would show phthalates and other plastics-related chemicals in a typical person's body. Asked whether exposure comes from food or water, she answers that it comes from all of the above.

To show how phthalates move from plastic into people, she describes a neonatal intensive care nursery. A baby receives food, nutrients and oxygen through soft plastic tubes. The baby metabolizes the phthalates, they pass into the urine because they are water-soluble, and researchers can measure them there.

the amount of phthalate that's in that urine is exactly proportional to the number of lines coming into the baby

She uses this to explain food contamination. Milking machines and other processing equipment contain the same soft plastic. Phthalates can also come from packaging and from cooking or microwaving in plastic, and she adds that they can be present in pesticides. Because phthalates are not chemically bound to the plastic, she says, they move into food, into people, and into a pregnant woman's womb, where they affect the offspring. She says she has studied this for about 20 years.

When the host asks which plastics are safe, Swan qualifies her answer. She calls it a changing landscape as new materials are introduced, notes that different plastics differ, and says she cannot speak to all of them. She offers an old rule of thumb based on recycling codes, which she believes is still mostly true:

four five one and two all the rest are bad for you

The rule has a limit she acknowledges herself: the wrappings on produce such as peppers usually carry no number, and she says the situation really varies. Her firmest advice is about heat. She says anything that comes in plastic should not be microwaved, and she recommends moving food to ceramic or glass and getting plastics out of the kitchen where possible.

Swan explains that the first evidence came from animals. Around 2000, researchers fed pregnant rats food contaminated with phthalates and examined the offspring. Exposed males were born different from unexposed males. To explain why, she describes normal development: early in pregnancy the genitals are a single undifferentiated ridge in both sexes, and in rats, 15 to 18 days into gestation, the testicles begin producing testosterone, which signals male-typical development. She stresses that this requires testosterone at exactly the right time and in the right amount.

A key measure in her research is anogenital distance (AGD), the distance between the anus and the genitals. She says it has been measured in animals for about 100 years, first to sort newborn litters by sex, because in males it is 50 to 100 percent longer. She argues that no other body measurement differs that much between the sexes in size, and that this holds for almost all mammals, including humans, with the hyena as an exception because its females are masculinized.

In exposed male pups, she says, the penis, the AGD and the scrotum are smaller, and the testes may not descend. The process of masculinization is arrested partway. Scientists in the National Toxicology Program named this cluster of changes the phthalate syndrome.

that pup is incompletely masculinized

The host asks whether people reach the exposure levels that produced these effects in animals. Swan says they do, and describes how she came to study the question. On a flight to a conference in Japan, a CDC chemist told her that the CDC was finding phthalates in everyone, including pregnant women, and that animal studies had shown effects on male newborns.

Her method was to measure exposure in the general population rather than start with children who had visible problems. Because phthalates are water-soluble, she measured them in urine. She notes that other chemicals, such as flame retardants, would be measured in blood. She recruited a population of pregnant women, measured the phthalates in their urine, and then examined their children. Since nobody had yet translated the rat genital examination to humans, her team developed one.

She published the first human evidence in 2005 that the mother's phthalate levels altered her son's genital development, followed by more work in 2008. She then secured NIH funding to repeat the study, noting that the agency does not usually fund replication and that these studies cost around five million dollars. The second study was more precise: urine was collected at three points in pregnancy and the babies were measured exactly at birth. The finding held.

now there's no question i don't think anyone questions that at least this class of chemicals which we know lower testosterone alter the development of these boys

Infants have no sperm count, so Swan used AGD as a bridge between the two. In rats, AGD appeared to be permanent, and she quotes toxicologist Earl Grey saying that AGD is forever. She adds an explicit caveat about humans: