Most EPA safety estimates, like the one for DBP, are based on data from testing in animals. The calculation for a safety level -- which is called a Reference Dose, or RfD -- starts with basic toxicological research, which involves dosing rodents at higher and higher levels until an effect is seen. Scientists work downwards from those high dose levels, determining the highest dose that causes no effect in rodents. This no-effect level is then divided by a factor of to 0 to provide an additional margin of safety. When this work was done for DBP, this procedure originally resulted in an RfD of micrograms per kilogram of body weight per day. For an adult male, that amounts to less than 3 ten-thousandths of an ounce a day.
Special interest groups, currently pressuring nail polish makers to drop DBP from their formulations, questioned the RfD of , claiming it was based on old data and might be outdated. EPA's review of the current science, which recalculated DBP's safety level using the latest and most reliable rodent studies, squarely answers this question. The draft of EPA's review now sets the reference dose significantly higher -- at 300 micrograms per kilogram per day. EPA says its new RfD for DBP merits a "high confidence" rating.
To put the new, higher safety level in terms of risk, the U.S. Centers for Disease Control and Prevention says that, based on the most recent biomonitoring data, the average daily human exposure to DBP is less than one microgram per kilogram per day. This is 300 times lower than EPA's new RfD.
In the course of updating its DBP safety level, the EPA also reviewed recent studies that report a statistical connection between phthalate exposure and human biological effects. One of these studies was the much-ballyhooed study by Dr. Shanna Swan claiming to show effects on male infants resulting from maternal exposure to a group of phthalates including DBP. EPA's review slaps another big question mark on the Swan study, finding "limitations" regarding the measurements used and the way maternal phthalate exposure was measured, among other problems, which in its opinion made Swan's work unsuitable for drawing any conclusions.
"If you look across the spectrum of reviews of DBP," said Marian Stanley, Manager of the Phthalate Esters Panel of the American Chemistry Council, "the news is good. A scientific risk assessment conducted by the European Union found 'no concern for consumers using nail polish containing DBP.' A U.S. review studying all uses of DBP found 'minimal concern" for pregnant women with average exposure. A government sanctioned scientific review found DBP 'safe as used' in current applications." So why is it banned from nail polish in Europe, and under attack in the United States? "Your guess is as good as mine," said Stanley. "We certainly know what it's not based on. It's not based on science."