As if it weren't bad enough that clueless environmental authorities try to impose their ignorance on the field by using so-called "metric" units of gas concentration, another trap may be lurking for you.

Everyone around the world agrees that a part-per-million (ppm) equals 10^-6. Unfortunately, since everyone around the world does not agree with what a "billion" is, they can't agree on what a part-per-billion (ppb) is, either.

In what is termed the American system, 10⁹ is a billion. But, in the European system, 10⁹ is a milliard (sometimes called thousand million). In the Euro system, a billion is 10¹². Ironically, the French used what is now the American system as far back as the seventeenth century, before switching to the Euro system in 1948. Are you confused yet?

While I have never encountered the term parts-per-milliard to signify 10^-9, it certainly could happen. Imagine if that were also abbreviated "ppm"!

However, the unit µl/M³ [microliter per cubic meter] has been published in numerous official Korean documents, and was used to avoid the potential parts-per-billion confusion. This interesting unit does define 10^-9 in a non-ambiguous fashion.

The good news is that American cultural hegemony has already affected the numbering system, and, as you might expect, money was involved.

We prevailed upon (at least) the Brits that 10⁹ dollars be called a billion dollars, and this was agreed to in 1974, under Prime Minister Harold Wilson. This usage has also spread into common parlance, and according to the The Times of London style guide,

[a] "billion [is] one thousand million, not a million million"

Even though our definition of a billion as 10⁹ is supposed to apply in official government statistics worldwide, the Korean example is clearly an exception, and is probably not unique.

Our recommendation is to ask what someone means by parts-per-billion, in any international dealings.