First, sorry for bad English.
If the fusion become a viable energy source, and used extensively, it will have impact on the amount of hydrogen in our environment? Unlike pollution by carbon dioxide, turn hydrogen into helium and it can not be broken again, photosynthesis is the molecular level, fission with hellium? then we would not be destroying the source of life on the planet? Creating more and more unbreakable Helium?
Nuclear fission has its radioactive waste and would never be feasible for home use because of the radiation, but dreamed cold fusion could move cars without immediate impact on the environment.
If what do I think is right, millions of cars creating helium would surely kill the planet in a few centuries.