When you call, National Grid comes out to measure the leak and give it a grade:
Grade 1 leaks are potentially explosive. They must be repaired immediately.
Grade 2 leaks are considered non-hazardous, but could become hazardous in the near future.
Grade 3 leaks are considered non-hazardous but National Grid charges ratepayers for the lost gas.
Grade 3 SEI leaks are super large volume leaks that have Significant Environmental Impact (SEI) because methane drives climate change. Some have been leaking for years!