Something that is harmless does not have any bad effects, especially on people's health.
This experiment was harmless to the animals.
这个实验对动物无害。
Scientists are trying to develop harmless substitutes for these gases.
科学家正努力开发这些气体的无害替代品。
If you describe someone or something as harmless, you mean that they are not important and therefore unlikely to annoy other people or cause trouble.
He seemed harmless enough.
他似乎不会惹麻烦。