Russel,
Take a deep breath (preferrably where the radon levels are low). Radon is not a black and white subject. There are several things to take into consideration before you make your decision on what to do.
1) Your Realtor is wrong when she says "the house being closed up for so long is causing high readings . . ." As a matter of fact those would be ideal conditions for conducting a radon test.
Radon is radioactive and has a half-life of about 3.8 days. To make the math easier in my explanation I will use a half-life of 3.5 days. If you had a certain amount of radon in the house on Day 1 then 3.5 days later 50% of that radon would remain and 50% would have decayed away. On Day 8 (7 days or two half-lives later) 25% of that original radon would remain. On Day 15 only 6.25% of the original radon would remain. After 4 weeks (on Day 29) only about 0.4% of the original radon would be left. This means the radon in the house cannot "build up" over time because the old radon decays away. After a month practically all the original radon has decayed away.
During that time new radon would have entered the house to replace the radon that has decayed away. After only about 12 hours the house reaches equilibrium where the new radon entering the house nearly equals the radon decaying away. So it does not matter if the house has been vacant for one day, one month or one year; the radon will not "build up" to higher than normal levels.
2) "Airing out" the house before another radon test will have little to no effect on the results. If the seller is determined to "air out" the house before conducting a second radon test you would want to make sure that closed-house conditions were maintained for at least 12-hours before the radon test was started. You also want to make sure the radon test is performed by a qualified radon measurement provider who has a certification from either NEHA or NRSB.
3) You also should understand that a short-term radon test is only an indicator of whether the house has the potential for elevated radon levels. Your 49-hour measurement of 4.3 pCi/L tells you that the house had slightly elevated radon levels under the conditions that existed during the 49 hours during which the radon test was conducted. Is this 49-hour measurement representative of your yearly radon concentrations? Nobody knows. The radon concentration in a house is constantly fluctuating. Personally I have measured the same house at the exact same location and gotten vastly differing results (one result of about 3 pCi/L and another of over 20 pCi/L) due to seasonal fluctuations.
Read what EPA says in their booklet 'Home Buyer's And Seller's Guide to Radon:
Interpreting Radon Test Results
. . .
Sometimes short-term tests are less definitive about whether the home is at or above 4 pCi/L; particularly when the results are close to 4 pCi/L. For example, if the average of two short-term tests is 4.1 pCi/L, there is about a 50% chance that the year-round average is somewhat below 4 pCi/L.
If I had done your radon test (49 hours with results of 4.3 pCi/L) my recommendation to you would be to consider doing a long-term test (91 days to one year) to get a better idea of your yearly average radon concentration. To make an analogy, if you fill up the gas tank of your car, drive it 100 miles, fill it up again and calculate your gas mileage you will get an idea of your car's gas mileage but it will not be very accurate (due to very little data) and may have been influenced by unusual conditions (traffic, wind, etc.) But if you calculate your gas mileage over several months you get a much more accurate and realistic number. Similarly a LT (long-term) radon test will give you more accurate and realistic results.
A big difference between ST (short-term) and LT radon tests is a ST test is a measurement of a home's potential for radon (which is why EPA protocols require closed-house conditions) whereas a LT test is a measurement of the radon exposure of the occupants under normal, lived-in conditions. Long-term testing is normally preferred if time permits.
Let me offer you a suggestion: Get a quote for mitigating the house and have the seller put that amount in an escrow account. After you buy the house you can have a LT radon test done. If the results are 4.0 pCi/L or higher you spend the money to mitigate the house. If the results are below 4.0 pCi/L the money if returned to the seller.
