In Papua, Indonesia, we routinely carry petrol, diesel and JetA DG loads. The drums used for transport are of all sizes, up-to and including 204litre (the old 44's). Most of the drums are not new, many of them having a service-life of potentially several years -and it shows. Additionally, the O-ring bung seals are a valued "personal wrist decoration" almost everywhere.
Harsh experience has taught a few lessons:
1. Plastic shopping bags do not an effective seal-replacement make! A plastic bag in a Toluene-rich environment has a remarkably brief service life! Therefore, if you see a plastic bag being used to replace a stolen O-ring, reject the load.
2. The drums are usually off-loaded by rolling them out the door onto an old tyre on the ground. Positioning of the tyre is critical both to "catch" the ballistic drum AND to prevent the drum being bounced-back into your airframe. It's a reasonably effective method, but inevitably does result in compounding, cumulative damage to the drums. Damaged drums subsequently leak. In the cabin. In-flight. A singularly unpleasant experience. Should I spot a drum in such a damaged state, or detect in-flight (smell, leaks) the drum is treated to a swift blow from the fire-axe to obviate further use. Harsh, but essential.
3. In-flight leaks are exceptionally dangerous, unpleasant and both time-consuming and difficult to properly rectify. The very best remedy is prevention. Ground-crews have to be trained to look-for and reject even minor leaks. Sometimes it's necessary to reject entire loads, to be certain the customer gets the message.
4. Fuel-drains have been known to jam open during drain-checks, showering anyone in the immediate vicinity in copious quantities of raw JetA. The Toluene content of the JetA will very quickly cause skin irritation, discomfort, rashes... it's Nasty stuff. If you or any of your crew are exposed, suspend ops until all personnel have had an opportunity to shower and change. It's one less thing you'll have to deal with in-flight, at a minimum.
5. Plastic drums -of ANY nature are banned in-flight under ICAO DG Regs (as I understand it)... yet they still appear here pretty regularly. Personally I refuse to carry plastic drums, under any circumstances.
I've been pretty extensively exposed to fume in-flight over the years from undetected leaking drums, I've known good friends that have had to stay on the ground several hours after landing to recover from fume exposure.
It's all too easy to rationalise and minimise the risks of leaks and exposure, when the pressure's on... do yourself a favour. Don't.