These days, new equipment and techniques allow the measurement of very small amounts. “Parts per” is the usual way scientists talk about contents, or concentrations, of say a pollutant in air or water. When scientists measure down to parts per million, billion or trillion, it’s important to have some sense of just how much – or how little – they are talking about.

A part per million may be hard to comprehend or visualize. Want to see how much it is?

It’s a credit card lying in the middle of a football field.

A step in a journey of 568 miles.

Or one minute in two years.

Taking that further, a part per trillion is a million times smaller than that credit card on the football field, for example, or 6 feet out of a journey of six round trips to the sun (Editor's note: 95 million miles one way to the sun).

When it comes to substances we take into our bodies by eating, breathing, or absorption through the skin, the comparisons are made relative to body weight. Normally scientists talk in terms of “milligrams (of material ingested) per kilogram (of body weight).” A milligram is one thousandth of a gram (a gram is about 3/100 of an ounce), and a kilogram is one thousand grams (or about 2 pounds). This is the equivalent of 1 part per million.

Want to see what taking in a milligram per kilogram of your body weight amounts to?

It’s the equivalent of 726 people, each weighing 150 pounds, sharing a chocolate bar (~50 grams).

Obviously, none of those people is going to get a very large piece of that chocolate bar. Now if that were an equivalent amount of a deadly poison such as cyanide, it could still cause harm. That’s why those tests on rodents are done – to reveal how much of a particular substance it would take to cause a negative health effects, at least in rodents. Are the results of rodent testing relevant to humans? Sometimes, no, but in the absence of evidence to the contrary, health officials assume the answer is yes.

Where do phthalates come in to this picture? To take a heavily publicized example, some have raised concern about the health effects of exposure to phthalates in nail polish. On average, nail polish contains 5% of a phthalate called DBP. Even though rodents can experience negative health effects when they are fed very high levels of DBP, it has been determined that there are no such effects when administered at levels of 50 milligrams per kilogram (or 50 ppm) of their body weight, per day. In order to get that level of exposure from nail polish, a person would have to use almost 5 bottles a day and absorb every bit of the DBP in it! Not even a centipede has that many fingers and toes to cover!

Similar concerns have been raised about phthalate exposure through perfumes. How much perfume would a woman have to douse herself with, every day, to reach the level that has no effect on rodents, assuming she absorbs every bit of the phthalate?

Two quarts!

The Centers for Disease Control and Prevention, a U.S. government agency concerned with public health, recently measured levels of phthalate metabolites in people's urine. These levels can be converted to phthalate exposures, which were on the order of micrograms (that is, millionths of a gram) per kilogram - or a thousand times lower than the levels that caused no effects in rodents.

The important thing to remember is to keep numbers in perspective. Big numbers don't necessarily mean big problems....or any problems at all.

Last Updated: July 16, 2003