Today’s scientific equipment and techniques allow the measurement of very small amounts of particles. “Parts per” is the usual way scientists talk about contents, or concentrations, of a chemical 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.

One step, on a 568–mile walk.

Or one minute, in a two-year span.

Taking that further, a part per billion is one thousand times smaller than that credit card on the football field, for example, or one heartbeat in 27 years for an average male (70 heartbeats per minute).

When it comes to substances we take in when we eat, breath, or absorb something through our 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). A milligram of a material ingested per kilogram of body weight 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.

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 might still cause some 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 effect, at least in rodents. Although the results of rodent testing may not always be relevant to humans, they are generally used as a basis for establishing safe levels of exposure to a substance.

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.

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