Gas Venting Stations: What's The Big Deal? On sectional charts you can see that, through various restrictions, regulators encourage pilots to avoid flying too close to Gas Venting Stations. These seem to be places where natural gas may occasionally be vented into the air (for example from pipelines). I have been unable to determine if the gas is preignited.

Is the principal risk from gas discharge that's being mitigated here:


aerodynamic
explosive/thermal
respiratory
a combination of the above?


Have incidents occurred relating to these risks?

You can find some talk of the risks online but it seems mainly to be in the form of "My mate told me...".
Gas venting stations sometimes release natural gas at extremely high pressures, so there is a risk of strong updrafts or turbulence. In any case if you fly directly over you may be going through a cloud of combustable gas which could possible ignite. It's a low risk, but given it's rarely a problem going around them it would be my advice to do so. 

I've flown over one without realizing and it put me in a 1200ft climb, I had to cut throttle in a hurry and dive to stay out of the London TMA. After that I paid attention to them.