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Different countries around the world use different measures of alcohol in breath.

In New Zealand, we use micrograms of alcohol per litre of breath: in a given litre of breath, how many micrograms (one millionth of a gram) of alcohol does the breath contain?

The symbol for this is μg/L. The current drink drive limit is 250 μg/L, and the DIC (criminal) limit is 400 μg/L.

Most company policies state cut-offs of 0 μg/L or 100 μg/L, with 0 μg/L being the most common.

New Zealand is fairly unusual, using μg/L. The only other countries around the world that use this are Botswana and the Netherlands!

The alternative that we see most often is grams of alcohol per two hundred and ten litres of breath: in two hundred and ten litres of breath, how many grams of alcohol does the breath contain? This is commonly used in Australia and America.

The abbreviation for this is g/210L. In this measurement, our current drink drive limit is 0.052 g/210L, and the DIC limit is 0.083 g/210L.

The updated standard for breathalysers, AS 3547:2019, specifies that breathalysers should measure in g/210L. This is a real pain for New Zealand – we have built up intuition over years of testing that 20 μg/L is a low result and 500 μg/L is a high result. That intuition suddenly no longer applies when being faced with readings like 0.0042 g/210L and 0.105 g/210L!

It remains to be seen how New Zealand will respond to this standard: whether we adopt it entirely and switch measurement units, or whether we adopt it in part and keep using μg/L.

If our measurement ends up changing, the magic number to convert between measurements and get an approximate result is 4,762. To get from μg/L to g/210L, divide by 4,762:

250 μg/L ÷ 4,762 = 0.052 g/210L.

To get from g/210L to μg/L, multiply by 4,762:

0.08 g/210L × 4,762 = 380 μg/L.