A C172 pilot attempted to fly to his destination airport despite the existence of a large fire and visibility reduced by the associated smoke. As he neared the airport smoke entering the cabin impeded his ability to navigate and was affecting him physiologically. At that point ATC provided assistance to direct him to his alternate.

Narrative

I had diverted to my alternate due to a large swamp fire near my original destination. However; as I was approaching the alternate it seemed possible to get to my destination after all. This decision was based on my ability to observe the ground lights and other identifiable geographic elements that were in my path and the reported ceilings were also above minimums.Approach vectored me but as I neared the airport it wasn't the visibility that became the problem but the smoke which was entering the cabin and making it difficult for me to maintain my focus on the instruments due to coughing (I'm allergic to smoke); a runny nose and watery eyes. Approach had agreed to stay with me until I was certain that I could make the runway. At one point Approach warned me that they had a LOW ALTITUDE warning for my aircraft. They then asked me how I was doing. I indicated it was not good...that I needed help getting out of there and back to my alternate which was in VMC. Approach vectored me as requested and I landed without incident.