What does the < symbol mean?

The “<” symbol indicates that the measured radon concentration in your sample was less than the detector’s Lower Limit of Detection (LLD). This is usually for one of two reasons:

1. Your radon concentration was very low and is below the detector’s LLD for these test parameters.

                                        OR

2. Your radon test kit took more than the recommended 10 days to arrive in our laboratory. The longer it takes to arrive in the laboratory, the higher the detector’s LLD is. If your test took more than 10 days to reach our laboratory, this will be notated on your test report in red.

What do I do next?

  • If your test result was <2.0 pCi/L (74 Bq/m3) or lower, you do not need to take any further action at this time.
  • If your test result was <3.0 pCi/L (111 Bq/m3) or lower, you may want to consider retesting, since your results are close to the EPA’s action level of 4.0 pCi/L (148 Bq/m3)
  • If your test result was <4.0 pCi/L (148 Bq/m3) or higher, you will want to retest. Your results may be at or above the EPA action level of 4.0 pCi/L (148 Bq/m3). To obtain a more precise measurement, ensure your retest arrives at the laboratory within 10 days.

More information on Lower Limit of Detection

For every test, the LLD is calculated. It primarily depends on the amount of time it takes for the test kit to arrive in our laboratory. Under ideal circumstances, our detectors are capable of measuring radon levels as low as 0.3 pCi/L (11 Bq/m3).

However, the longer the kit takes to arrive the more the radon in the test kit undergoes radioactive decay. This means that over time, radon and its daughter particles “disappear” from the test kit. As these particles decay, the LLD rises as there is now less radon in the test kit to measure. This decay of radon raises the Lower Limit of Detection.

Because we want to provide you with the most accurate test result possible, if the measured radon level is below the detector’s LLD, we report the test result as Less than the LLD value. We can know with a high degree of confidence that the radon level in your sample was below the LLD. However, due to statistical variation and radioactive decay, we cannot precisely pinpoint an exact radon level with a high degree of confidence.

For example, if the measured radon level in your test kit was 1.0 pCi/L and the LLD for your test was 2.0 pCi/L, we would report the result as “< 2 pCi/L”. In this example, it is equally likely that your radon level was 0.1 pCi/L or 0.5 pCi/L or 1.0 pCi.L or 1.9 pCi/L, or any other number below 2.0 pCi/L. Because the measured level was below the LLD (in this case, 2.0 pCi/L), any measured radon result below 2.0 pCi/L could be simply due to statistical variation. It is impossible to know the exact level with a high degree of confidence due to statistical variation.

We can know with a high degree of confidence that your radon level was below the indicated value. However, we will be unable to precisely state the exact radon level present. To ensure you have the most accurate results, we will report the test result as Less than the LLD Value. 

In the event that your test kit takes a long time to arrive in our laboratory, we will still do our best to calculate a valid radon result. However, when the test takes a long time the LLD value can rise substantially. For test kits that arrive in 10 or more days, the LLD value can become very high. This is the reason we recommend using expedited shipping to return the test kit to the lab, and the reason we must receive your test within 10 days for best results.

 Question & Answer

Q: Does this mean my result isn’t accurate?
A: Your test result is still very accurate, however, the radon level is simply lower than the Lower Limit of Detection. This means that we cannot precisely pinpoint an exact radon level, but we can say with a high degree of accuracy your radon level is below the number stated on your report.

Q: I sent in more than one test. One of my results has a “<” symbol but the other doesn’t. Why are they different?
A: The LLD primarily depends on the amount of time the test takes to reach our lab, but it depends on other factors too. Each test is unique and has slightly different characteristics. The LLD depends on the individual test measurements, as well as the characteristics of the analysis equipment used to analyze your test. Since no two tests are identical and since we have multiple sets of analysis equipment in our lab, even two nearly identical tests will often have different LLD values. This is normal and expected and not a cause for concern.

Q: I sent in more than one test. One result had a “<” symbol but the other test was an error. Why did this happen?
A: Because the LLD is different for every test, it’s possible for one test to be valid and another to be invalid. Any test where the LLD is more than 4 pCi/L is always reported as an error. This is because the EPA’s action level for radon is 4.0 pCi/L, meaning the EPA recommends you take action to address a radon problem if your level is about 4 pCi/L. Since the action level is 4 pCi/L, a LLD above 4 is not helpful information – a test with a result of < 5.0 pCi/L could represent a low level of radon, or a high level of radon above the action level. Since this information isn’t useful to determine what to do next, we never report a test result where the LLD is above 4 pCi/L. This means you will never see a test result that looks like “<4.2 pCi/L” or “<5.1 pCi/L”, ect.

Q: I need more help understanding the LLD and why it matters for my test.
A:  Let’s compare this to a situation that’s easier to understand. Imagine you have a bucket full of water. Let’s say there are 2 gallons of water in the bucket. There is a hole in the bottom of the bucket, and water starts draining out of the bucket. Every hour, 1 gallon of water drains out of the hole in the bucket. You walk over to your neighbor’s house with the bucket. It’s a very far walk and it takes 3 hours to get there. By the time you arrive at your neighbor’s house, there’s no water left in the bucket, since it’s been 3 hours and water drains from the bucket at a rate of 1 gallon an hour.

Your neighbor wants to look at the bucket to figure out how much water there was in it at the time you left your house. All your neighbor can see is that the bucket is empty. You tell him it was a three hour walk and the water drained at a rate of 1 gallon per hour. Your neighbor can now determine that there were Less than 3 gallons of water in the bucket at the time you left your house, because in the amount of time you walked over no more than 3 gallons of water could have drained out of the bucket.

However, your neighbor can’t tell you exactly how much water was in the bucket. They know it was less than 3 gallons. If it has been 2 gallons of water, the bucket would still be empty when you arrived. If it had 1 gallon, the bucket also would have been empty. If you had left your house with an empty bucket, the bucket would still be empty when you arrived at your neighbor’s house. If you had left your house with 4 gallons, there would still be 1 gallon left in the bucket after 3 hours, and your neighbor could figure out exactly how much water was in the bucket when you left your house.

As you can see, since the bucket arrived empty at your neighbor’s house, they can’t figure out exactly how much water you started out with. Using math, they can know very accurately that there was less than 3 gallons of water, but they can’t know precisely how much water.

This is very similar to how a radon test works. The ‘bucket’ is the charcoal in your radon test kit. It adsorbs radon gas in your home. The radon gas is the ‘water’ in this case. When you seal up the test kit to send it to our lab, the radon gas in the charcoal starts to decay, or ‘drain out of the bucket’. When the test arrives in our lab, we measure how much radon is still in the charcoal, or ‘how much water is still in the bucket’. But, we want to know how much radon was in the test kit when it was in your house a few days ago. So, we use math to try and figure this out, since radon ‘drains’ from the ‘bucket’ at a known rate. If your test kit doesn’t have much radon in it, or if it takes a long time to get to our lab, the ‘bucket’ is ’empty’ when we measure it here in our lab.