CNG is affordable and produces way fewer emissions than a petrol/diesel car. However, there are a few rules one has to observe at a CNG pump that may not make much sense at first glance.

So, why is it best to step out of your car while filling CNG? 

That's because CNG, by nature, is volatile, and you cannot predict what happens next. It may heat up, catch fire due to leaks, or pressure burst, and there are many more reasons, but it all comes down to the fact that your vehicle may ignite.

Apart from this, the meter of the CNG pump is also different as compared to petrol and diesel. To avoid any rigging with customers they are asked to get out of the car to check the meter. 

Also, the smell of CNG can make you feel giddy and cause inconveniences in your travels, so it's best to step out of the vehicle. 

Please note: Stepping outside the vehicle does not mean you stand outside the car door. If you are with your family, ask them to stand out from the petrol station.  

Here's what a Quora user had to say about it - CNG kit in your car is a charge-discharge system for gas. While refilling, more than 200 bar of pressure is pushed through lines inside your car. This pressure can make any existing crack bust open, can cause fire due to leaks, or pressure bust and cause misery so better prevent it.

You're welcome.