I'm allergic to cats to the point that when I was an infant they didn't have a number on their scale to quantify it. When I'm exposed to cats, my eyes run and swell shut, my skin gets itchy, but above all that my asthma goes well into problem territory. I imagine there are people allergic to dogs in a similar way, and being in an enclosed space with a dog for multiple hours may, in fact, be a threat to their life.