It is scientifically proven and well-known that the gas released by cars is toxic and quite dangerous to the environment, especially to the ozone layer.

scientifically 是由名词 science（科学）加上后缀 -ific（表示"形成...的"）变成形容词 scientific（科学的），再加上副词后缀 -ally 构成的副词，表示"以科学的方式"或"从科学角度"。在句中修饰"proven"，强调这个结论是通过科学方法验证的。