“Reduce blood levels and see if functions return” so like reduce smoking to see if lung cancer go into remission? Why the assumption health effects are reversible?
You can certainly reduce smoking to see if lung capacity improves. I don't think it's reasonable to reduce smoking to see if that will cause an existing tumor to go into remission, for the same reason it's not reasonable to see if installing a blade guard on a saw will make a amputated finger reattach or regrow. The risk has manifested, and removing the risk factor is too late.
You'd need a large study to determine if stopping smoking reduces future risk of tumors; it's not something you can determine in a single person.
Not all health affects are reversible. Some reversals can be quite evident for acute symptoms, like arsenic. IQ reduction from lead appears to be permenant. Using your example of lung cancer... no, the cancer would not go into remission if established. However, there are studies showing lung cancer risk go down for smokers after they stop smoking, and generally the risk continues to decrease the more time has passed since they last smoked.
You'd need a large study to determine if stopping smoking reduces future risk of tumors; it's not something you can determine in a single person.