Reisen is just plain impure now, though. LoLK dialogue confirms it, but when you're outright living on the earth (not just stopping by for a short visit) it's an inevitability.
To make things simple, Eientei people basically live the same way Earth people do ; picture this :
>Life + death = Impurity
>Life & death = two sides of the same coin
>Life leads to death
*Daddy, how does life work ?*-Well, you're born, you grow old and you die, oh and you suffer in-between btw
-Lol k
>There's growth involved
>Growth takes time
Meaning !
[Birth]===(kid)===[Growing up]===(adult)===[Growing up]===(old age)===>[Death]
>Growth leads to death
Logically Obviously, growth / passing of time bring people (well, normal ones) closer to their deaths, it has to play a role in the whole impurity thingy, since you
usually don't go straight from birth to death (that'd be nasty).
Take Eientei for example : back when it was under Kaguya's spell of eternity, food wouldn't go bad, dust wouldn't gather, the whole building wouldn't need maintenance, Kaguya's Udonge bonzai wouldn't grow... basically, when Eientei was pure, it was unaffected by the passing of time, it didn't "grow old", so to speak. When Kaguya released her spell, Eientei popped into history, it became affected by the passing of time... like every other building out there : it pretty much became "impure". Just saying, but although they are supposedly "impure" (/ growing / feeling the passage of time (?)), it's not like Kaguya's Udonge bonzai magically bloomed all of a sudden, it just continued to grow.
Basically, nasty things happen to you because you're impure, although that shouldn't be that an issue for those with long life spans, though. If you're "impure", you're just living like ordinary Earthling, with everything this implies.