Sentence:  Since the mid-1980s, when Indonesia first began to clear its bountiful forests on an industrial scale in favour of lucrative palm-oil plantations, “haze” has become an almost annual occurrence in South-East Asia. The cheapest way to clear logged woodland is to burn it, producing an acrid cloud of <WSD>foul</WSD> white smoke that, carried by the wind, can cover hundreds, or even thousands, of square miles. 
Meaning of the word marked with <WSD> tags:  Covered with, or containing unclean matter; dirty.