這裡是“釋放”的意思，“灰塵、煙塵及有害的化學物質被釋放出來”，所以選let out.

他們的具體用法如下，供你參考：

1、let off

讓…下車[下船]; 引發，引爆; 放掉; 免除;

This train stops to let off passengers on signal.

這列火車按訊號停車讓旅客下車。

They let off their guns at the British ships.

他們對英國船舶開炮。

The janitor let off some steam because the pressure was too high.

鍋爐工把蒸汽放走了一點,因為壓力過高。

2、let out

放出; 釋放; 發出（聲音）; 使（火）熄滅;

The Doberman let out a string of roaring barks.

短毛獵犬一陣狂吠。

She heard him let out a pitiful, muffled groan.

她聽到他發出一聲令人悲憫的低沉的呻吟聲。