what is PPM and why gas used to measure in PPM?

ppm is an abbreviation of parts per millionppm is a value that represents one ten thousands  of one percent in total air volume

It's the same reason why, for example, we measure people's weight in pounds and not tons. It's easier to read and express things in ppm when you're dealing with measures that are much less than 1%, as "percent" literally means "per hundredth". 

If something had "0.5 parts per million", that would be a fraction of 5 ten millionths, or 0.0000005, which is 0.00005%. Having all those zeroes in there could make it easy to misread it as "0.000005" or "0.0005". So rathan than spending so much time keeping track of zeroes, it's easier to just express some percentages as "ppm".

1 ppm=0.0001%
1%=10,000 ppm

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