Just to follow up on that call to SA, as promised:
Basically, if you have your allergy noted in your PNR (consider calling SA for this) if there are any Assistance Dogs onboard, you should be notified at check-in. There are options then, regarding re-seating in the same class of travel and further away from the dog. However, if the re-seating options aren't acceptable, then travel can be booked on the next available flight to the destination for the allergy sufferer - note that the person requiring the Assistance Dog won't be offloaded.
Regarding air recirculation, I think the figures mentioned were something like each pax receives ~22 cubic litres of air, 14 of which are fresh and the air in the cabin is completely recycled 30+ times per hour. The recycled air will have been passed through HEPA filters which can take out particles up to 1/10000th of the width of a human hair. The air comes in via the overhead vents and is extracted through floor louvres, so it doesn't travel longitudinally and therefore, if you're more than ~3 rows away from the dog, you're unlikely to be affected.
CC are under instructions not to pet or touch the Assistance Dog, so as to limit the spread of dander, and the dog should have its own toilet facilities (I believe the owner arranges some equipment to carry).
VS do recommend that allergy sufferers carry antihistamines, an epipen and/or piriton syrup. They also note that they haven't carried any dogs under this scheme as yet, there are some pretty stringent requirements to be followed (with the onus on the owner) if the dog is to be carried, and VS need 14 days notice - see [url=http://www.virgin-atlantic.com/en/gb/passengerinformation/assistancedogs
/index.jsp]here[/url]
I certainly feel more confident about being onboard with one of these dogs after the conversation. Although I was the very first query on the subject, it was a question that had been anticipated to some extent, and I was glad of the opportunity to discuss arising issues - indeed I think the very helpful chap on the end of the phone was glad of the discussion too
(I also offered to assist if they needed any information passing via this site)
HTH
Cheers
Michael
Anyway, there's quite a few measures in place, lots of paperwork to be done, etc.. etc... - also had the air recycling system explained, which was very interesting & reassuring. I'll flesh this out with some more detail later, but suffice it to say that I'm happy that VS have really thought about this and I think it'll work out ok.
