The following is written as information only. The following in no way constitutes any form of accurate legal advice. If you choose to use Hydrogen the onus is on you to research the implications and take appropriate safety measures both for yourself personally and the general public.
Most Helium seems to be produced as a distillation from natural gas – Its not clear if it would be vented if it was not captured.
Hydrogen has a density of about 0.0899Kg/cu m (at 1 atm 15C). Pure Helium has a density of 0.1786Kg/cu m however balloon grade helium is usually only 97% Helium – giving it a density of 0.2094Kg/cu m. Air has a density of 1.225Kg/cum – giving:
In practice the difference between Helium and Hydrogen is much less in terms of altitude attained. For example a 1Kg payload on a HY-1000 balloon at 5m/sec ascent rate would typically achieve around 32.0Km (105,000ft) on 97%Helium and 33.1Km (108,000ft) on Hydrogen – so only 3% better.
Although Hydrogen is cheaper to produce its end cost to HABers can be very similar to Helium.
There are several re-sellers of Helium - prices are about £20 (inc VAT) per cubic meter (based on Air Products N20 300bar cylinder 3 months free rental).
Hydrogen prices work out about £16 (inc VAT) per cubic meter for a similar sized container (BOC 300bar G20 – 1 moths rental).
Both of the above are lightweight (25Kg) cylinders that can be easily maneuvered. Hydrogen prices drop to about £6 per cubic (inc VAT) if you are prepared to have a 70Kg 9cu m cylinder (based on Air Products 200bar X47 – 30 days rental)
Prices from early 2018
The most likely cause of ignition for HAB use is probably a static discharge between the filled balloon and personnel or structures. During Operation Outward (WW2 launch of circa 100,000 hydrogen met balloons) there were several instances of the balloons catching fire due to static discharge between the balloons and filling tents. In some cases personnel were badly burned (but no fatalities).