How I can find out the approx. level of carbon monoxide in my blood 24hrs prior to a blood test?

I have looked on many sites but I do not understand them. I had been exposed to a faulty boiler which was repaired on Wednesday morning. My doctor sent me for a blood test yesterday morning and has just telephoned me with the results. My blood contained 5.2 carbon monoxide, but I know that it can dissipate quickly once the source is found. He said that anything over a reading of 6 would be cled harmful. Is there someway I can find out what the reading may have been 24hrs before? Thank you.