Winco keeps a professional on the ground during the cleaning process to keep plants rinsed off in the rare event that our cleaner reaches the ground. Then we spray on our own custom algaecide cleaner in a thick foam, which both kills the roof eating algae and loosens up any soil on the roof. Once the solution is almost dry we spray on a mild harmless neutralizer which renders the cleaning solution harmless to plants. This neutralizer is actually used to make drinking water safe.
Then we use a garden hose to gently rinse off the harmless dirt and dead algae from the shingles. Our algaecide cleaning solution does contain some chlorine, but we are very careful to keep it were it goes during the cleaning process and to keep it rinsed off of any plants during the cleaning process. Chlorine quickly goes into a gas state once foamed on to the shingles. The air and sunlight send it airborne.
Almost every swimming pool in the world rids itself of chlorine everyday. The neutralizer takes care of the remaining residual, so that what comes down onto the ground is just dirty water. Chlorine is what most all of us drink everyday in our city water. We bath with it, cook with it, swim in it and clean with it. The key to using it, is a professional safety.