Dr. Joe Schwarcz, a legend in the field of science communication and the director of McGill's Office for Science and Society, did what so few journalists and commentators are willing to do when it comes to phthalates: look at the evidence and share this science-based analysis with readers of the Montreal Gazette. Following that approach led Schwarcz to a simple conclusion—as used today, phthalates pose little risk to human health, though we should always be open to new evidence as it arises.
The controversy begins
So how exactly did phthalates, chemicals used to make plastic more flexible, become the objects of so much negative press? Schwarcz offered a helpful history lesson:
“Concerns about phthalates began to be raised in the 1970s when they were detected in blood samples from patients who had been treated with intravenous fluids stored in PVC bags. When biomonitoring studies revealed that phthalates and their metabolites were detected in almost everyone’s urine, scientists quickly organized studies to explore the effects of these chemicals on health.
Experiments using cultures and animals revealed that phthalates, to varying extents, had hormone disrupting properties. This was corroborated by some observational studies in people that linked higher levels of phthalates in the blood of pregnant women to developmental problems in their male offspring, such as smaller penis size.”
These were concerning results that sparked legitimate questions about the potential risks of phthalate exposure. But, Schwarcz continued, there's much more to the story. Perhaps most importantly, the correlation posited in these studies was not evidence of causation:
“Let’s start by pointing out the obvious. An association does not prove a cause-and-effect relationship. A classic example is the correlation of ice cream sales with drownings. Ice cream sales increase in the summer as do drownings, but people do not drown because they have eaten ice cream. So while phthalates may correlate with early death, there may be a number of confounding factors.”
Confounding factors everywhere
But if phthalate exposure isn't to blame for these negative health effects, what is? There are probably several other variables that offer a better explanation, according to Schwarcz:
For example, most of the phthalates that show up in our urine come from our diet because food is exposed to all sorts of plasticized substances ranging from flexible packaging and machine parts to vinyl containers. People who have a higher calorie intake will therefore have more phthalates in their urine, and it could well be that it is the higher calorie intake and not the phthalates that leads to an earlier death.
Also, greater consumption of processed foods is likely to result in more phthalates in the urine, but it can also signal a higher salt intake and it may then be the salt that leads to higher blood pressure and premature death, meaning that a correlation with phthalates may just be spurious.
What the science does (or doesn't) say
When responding to misleading media coverage, we often stress that studies raising concerns about phthalates aren't nearly as conclusive as portrayed by the press. Schwarcz cited three important limitations in the early research on phthalates that restrict the conclusions we should draw from it:
“The urine samples were taken at just one point in time and it was that single phthalate level that was used to forge the correlation with early death. But phthalates are quickly metabolized and excreted in the urine, so urine levels can vary depending on when the last meal was consumed.
“Also, lumping all phthalates together can skew results because there are significant differences in the physiological properties of different phthalates, and the ones that have raised the most concern have been mostly eliminated since the ... samples in the study were taken.”
“On top of this, the actual correlation found was statistically quite weak, and importantly, when subjects with known cardiovascular disease at the beginning of the study were eliminated, the correlation between early death and phthalates disappeared.”
Schwarcz added important perspective in his summation:
“Lack of sleep, too much sitting and lack of proper hydration have also been linked to poor health outcomes so that targeting phthalates, or indeed any other single substance or lifestyle factor, as being specifically responsible for disrupting health is unjustified.”
The takeaway, as always, is that we should employ a little skepticism when reading headlines and scrolling through our social media feeds. Critical thinking is our best defense against the morass of misinformation that proliferates across the internet.