Hydrogen is produced and used in the hundreds of millions of tons, today. People say it is mostly produced on demand at the site of use because of the difficul

Hydrogen is produced and used in the hundreds of millions of tons, today. People say it is mostly produced on demand at the site of use because of the difficulty of storing and transporting it. If it becomes an important part of the energy economy, it will need great care in preventing unavoidable leaks from escaping into the atmosphere. Of course for most uses, hydrogen will continue to be produced on the spot, by electrolysis instead of from natural gas as today. In places where it is not used immediately (e.g. to synthesize ammonia for transport), it will often be stored deep underground where leakage is less a worry. As aviation transitions to liquid hydrogen fuel, probably after 2040, hydrogen produced at major airports may be stored liquified in underground tanks, at a pressure close to ambient. Hydrogen leakage is minimal at low pressure. In general, for safety, installations that handle much hydrogen must continually flush air through indoor places with hydrogen plumbing, to prevent buildup of what would be an exceedingly dangerous mixture of hydrogen and air in confined spaces. It might be that sites handling much hydrogen will need to feed such air into an incinerator. A way to recognize significant hydrogen leaks, as we have for methane, probably will be needed.