机外净化是指在汽车排气管上安装三效催化转化器，使尾气中的有害成分迅速转化为无害物质。

It refers to install the three way catalytic converter at the automobile exhaust pipe, and rapidly transform the harmful ingredients into innoxious substance.

科学家们正在努力使变形有害化学物质变成无害物质。

Scientists are working to transmute harmful chemicals into harmless ones.

过敏反应可能发生当免疫系统起反应对无害物质.

Allergic reactions can occur when the immune system over reacts to innocuous substances.

但有时候，免疫系统会不小心排斥无害物质，例如花粉。

But sometimes, the immune system accidentally discriminates against harmless substances, like pollen.