If someone rents a house from you and it turns out that the ducts are filled with black mold, you have a responsibility to your tenants to provide for their medical care. If they were living in the house next door, they would not be sick and would not need medical care. If however, someone rents a house from you and the city develops a smog problem, you do not have a responsibility to pay for their respiratory care. Even if they lived in the house next door, they would still have the same health problems.
I believe that if a landlord can prove that they had no prior knowledge of any pollutants in their home then they should not have to provide care for environmentally induced illnesses. I think this would place too high of a liability on landlords especially in cases where you could not prove that the illness was a direct result from the rental home. I think landlords and rentals should do their due diligence before renting.
I do not believe that any home that any home that is rented should provide care for environmentally induced illnesses. I do not think it is the responsibility of the owners to have to provide such care for something that isn't their fault. I think such a thing would harm the housing market.
I think the healthcare system should provide care for any environmentally induced illnesses. Do you understand how impossible it would be for land lords to provide this sort of care without the help of government? No, this is silly. It isn't up to them to provide this sort of care.
Most people who are renting homes have options when shopping for the place they want to live, and to imply liability by the owner on the part of environmentally induced illnesses would be unjust. I woudl believe it is rare to be able to 100% determine an illness was caused by some failing home owner or person who rents out a home.