hello Lets imagine that we have become IMMUNOTOLERANT to these dusts & pollen. Then our body / and out immune system won't mount any response against them.. isn't it? These particles would enter our airways & gradually over the years will get accumulated in our alveoli/ bronchioles/ smaller airways. Still our immune system doesn't clear them off by macrophages or mucus as we are immunotolerant...what will happen next? We will develop breathing problems( not over span of years, but within a few days as we have a lot of dust around).Our airways would get clogged....Not to forget about infections! Nature has given us a very simple way to deal with such issues- sneezing !  even if particles do get past by, there are many other ways to tackle them. Their absence would have lead to death of humans by even smoking their 1st cigarette!!