1% means 1 part out of a 100:

2% butterfat milk means that 2% is fat and 98% is something else, compared to skim milk which will be marked as 1% butterfat or less

Eventually we go down to 0.1%, 0.01%, 0.001%, but these numbers get kind of cumbersome, so it becomes convenient to change scale.

100% also means 1 million out of a million.

Therefore, 10% is the same as 100,000 parts of a million (ppm).

1% is the same as 10,000 parts per million.

This one is important, 1% = 10,000 ppm.

Let’s look at some common examples:

Oxygen in the air we breathe is 20.9% (or 209,000 ppm)

The time-weighted average (TWA) for carbon monoxide is 25 ppm (the same as saying 0.025%)

TWA for hydrogen sulphide is 10 ppm

Carbon dioxide in the air we breathe is around 500 ppm

Carbon dioxide in our exhaled breath is several thousand ppm

1% = 10,000 ppm

0.1% = 1000 ppm

0.01% = 100 ppm

0.001% = 10 ppm

0.0001% = 1 ppm = 1000 ppb

0.000001% = 0.1 ppm = 100 ppb

0.0000001% = 0.01ppm = 10 ppb

0.00000001% = 0.001 ppm = 1 ppb = 1000 ppt (parts per trillion)

When it comes to measuring combustible gases, we use a slightly different scale.

If we use methane (CH4) as an example, the LEL (lower explosive level) of methane is generally considered to be 5% by volume (5% bv). That means the mixture is too lean to burn if there is less than 5% methane present. But at 5%, we can burn or explode if there is an ignition source.

So we call 5% bv methane the same as 100% LEL. 5% methane is 100% of the explosive concentration.

Most portable gas monitors will alarm at 10% of the LEL. Regardless of whether we use the %LEL scale, the percent scale or the ppm scale, the concentration is all the same.

100% LEL = 5% methane = 50,000 ppm methane

10% LEL = 0.5% methane = 5000 ppm methane

1% LEL = 0.05% methane = 500 ppm methane

Pretty much all combustible gas monitors are built to be layman-proof.