And that was the day that they found a "havenworld" that was actually deadly to anyone not born there. -- Anon Guest
The Havenworld/Deathworld scale judges only how hazardous an origin planet is for its inhabitants. How many factors are in their favour or, in the case of Deathworlds, actively trying to kill them. For the Phyltrathi of Planet Fluff, their planetary abode was paradise. All they had to do was take a deep breath and the spores and threads in their air sustained them more than adequately.
It was quite a shock to them to discover that not all intelligent life were filter-feeders. It was an even bigger shock that other cogniscents didn't derive the same amount of nutrition out of their spores or threads that they did. Worse... it could have easily be taken as a poisoning attempt. Fortunately, more rational minds prevailed.
First, scans revealed a lot. They helped both sides solve a lot of problems before they became problems, let alone ambassadorial incidents. Necessity, the primary parental of invention, created the Spore Fogger, a means to limit the exposure of the toxic Phyltrathi foodstuffs to the Phyltrathi enjoying them. Once they dissipated to less than five parts per million, they were relatively harmless.