The most commonly used rocket fuel is hydrazine, which is extremelly harmful to the athmosphere. Hydrazine has both short and long term effects on the human body.
Short term effects include, but are not limited to, skin and eye irritation; headaches; dizziness; nausea.
Long term effects include, but are not limited to; seizures; comas; damage to liver, kidneys, and the central nervous system; pulmonary edema.
All of these effects, and more, can also apply to animals. The less hydrazine organisms breathe, the better.

The use of H2O2 at launch minimizes the amount of toxic rocket fuel that is used. Any little bit counts.
Too much of anything is bad, but it’s difficult to have too much H2O2. It doesn’t harm the atmosphere, and more of aquatic life. It sterilizes the water, which is good for humans but harmful for most fish. There was a study in Japan, stating that a few rivers were polluted due to H2O2. The highest concentrate of H2O2 in these rivers was >75%, but most of the rivers had less H2O2 than the PNEC.
This is a very rare occurrence, since peroxide biodegradable and usually dissolves in water. As long as water stays less than 0.7mg/L, there aren’t any negative impacts.