This delightful article raises the concern that the pollen of transgenic trees could trigger anaphylaxis:
Before leaving the discussion of the control of flowering in trees it may be useful to consider the potential side effects of the ablation toxins used to create sterile trees. Barnase ribonuclease proved toxic to the kidneys of rats (19). Barnase was cytotoxic in mice and in human cell lines (20). Animals may not find the GM forests welcoming. Diphtheria toxin has been associated with anaphylactic response (221).It's hard to imagine anything worse than the pollen allergies I already have, but it's nice to know that biotech firms are working on the problem.
I'd also suggest that the "need" to create these pointless trees, and to make them sterile, is symptomatic of the cognitive and ethical disarray endemic among biotech researchers.