"Disposable diapers linked to infertility" proclaimed USA Today earlier this week. "Diapers may harm testes: study" headlined the Chicago Sun-Times. "Do disposable diapers harm boys?" asked the Deseret News. Is it time to dispose of disposables?
No — unless you plan to substitute the diapers with the flawed scientific study underlying those news reports.
German researchers monitored the scrotal temperatures of 48 boys, ages birth to 4 years and 7 months, using a non-invasive thermal probe. They reported that scrotal temperatures of boys wearing plastic-lined, disposable diapers were about 2 degrees Fahrenheit higher than scrotal temperatures among boys who wore cotton diapers.
Because 2- to 6-degree increases in scrotal temperatures affect sperm development and motility in adults, the researchers hypothesize that infant boys who wear disposable diapers might have fertility problems later in life. And to give their findings currency, the researchers assert that "Male reproductive health has deteriorated in recent decades," coinciding with the use of disposable diapers. The study appears in the October issue of the Archives of Disease in Childhood.
Serious questions exist about the study's methodology. But the truly flaky claims are the assertion that male fertility is on the decline and that disposable diapers have played a role in the decline.
As to the study, even assuming the temperature measurements are correct, the researchers acknowledge that no one knows what scrotal temperatures in children normally are; much less do they know how infant scrotal temperatures affect adult fertility, if at all. These data gaps alone should have prevented the researchers from jumping to any conclusions.
The researchers inexplicably passed up the opportunity to explore "normal" scrotal temperature as they only measured temperatures in children wearing disposable diapers and cotton diapers. There were no "buck-naked" controls.
The test also didn't reflect real-world use of cotton diapers that are typically worn with plastic pants to prevent leakage, according to the Personal Absorbent Products Council, a business group.
The temperature probes were designed for adults, not babies. Infant testicles are small in comparison with the scrotum. So the probes may not have measured testicular temperatures, according to Professor Eberhard Nieschlag of Muenster University in Germany.
Temperature data were measured over two 24-hour periods for each infant. But the measurements were not made under controlled settings. Each infant was under the care of his mother in his own home. Temperature data was collected from the 48 infants over a period of one year, excluding July and August. There was no consideration of variability due to indoor or outdoor environment, or infant handling.
Finally with respect to the study, though higher temperatures are known to reduce sperm count in adults, the effect is reversible. And infants won't be making sperm until puberty starts anyway — probably 10 years after the diaper-wearing phase.
"This poorly done study raises a question based on information that is irrelevant, uses techniques that are not adapted to infants and children and reaches conclusions that are unfounded, " says pediatrician and UCLA clinical professor Dr. Lorraine Stern.
The real mess in this diaper study, though, is the exploitation of the unproven claims that sperm counts have declined over the last several decades.
"Study Points to Global Sperm Count Reduction," proclaimed the Associated Press in September 1992. Danish researcher Neils Skakkebaek claimed to have identified a 42 percent decline in mean sperm density and a 19 percent decline in seminal volume among 14,947 men studied in 61 papers published from 1938 to 1991. Skakkebaek's study started a furious debate over male fertility. The most publicized allegation is that exposures to manmade chemicals in the environment caused the supposed decline in sperm counts.
In the diaper study and the related media reports, the German researchers cite claims over male infertility as if the controversy itself constituted some sort of proof. They cite studies that seem to support their contention without any balancing information.
The opening sentence of the study reads, "It has been suggested that male reproductive health has deteriorated during the last two to three decades." This sentence is followed by a presumptuous non sequitur: "several studies have shown that the quality of human semen has declined."
Texas A&M University research Stephen Safe says, "Results of recent studies show that there are large demographic variations in sperm counts within countries or regions, and analyses of North American data show that sperm counts have not decreased over the last 60 years."
Fertility experts recently stated in the International Journal of Andrology, "To show that male fertility is declining is not simple. Few men volunteer and recruitment bias may lead to over-representation of the subfertile. Semen analysis has errors arising from counting and poorly standardized criteria." In other words, conclusions may be hastily drawn from data of questionable quality.
Sperm count claims cannot be easily disproved because of a lack of data and subtlety of effect. Perhaps that why they make the perfect allegation for opportunistic alarmists.
— Steven Milloy is a biostatistician, lawyer, adjunct scholar at the Cato Institute and publisher of Junkscience.com.
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