Thoron and radon remediation

Often, when a homeowner tests to see if a radon abatement system is needed, thoron levels are also divulged. However, thorium, which is similar to radon, is a shorter-lived element which decays about 6,500 times faster than radon.  In fact, thoron has a half-life of about 51 seconds. Radon, on the other hand, has a half-life of just over 92 hours. Therefore, since most of the thoron never makes it very far from where it originates, the US EPA does not consider it to be a major contributor to health problems.

Both radon and thoron are considered noble gasses. This means they are free agents in the soil and can easily move out of it and into your home.  However, the difference is that while radon has plenty of time to meander up through the soil once it is produced by radium’s decay in the soil, thoron only has a few minutes to do so.

Moreover, when thoron decays it becomes a solid reactive particle that easily clings to dirt in the soil and dust in the air and while the first decay product of thoron, Polonium 216, has a fairly long half-life of 10.6 hours and can be breathed into the lungs if it is airborne, because of its long half-life compared to that of Radon decay products, the lungs can push it back out before it decays.  Therefore, these two factors of not having enough time to get out of the soil and into a home and the long decay life of the first decay product have led to thoron being placed in the low-risk category when it comes to lung cancer.

However, bear in mind that there is some potential of over-estimating the amount of radon in a measurement by inadvertently including the thoron decays.  Therefore, it is recommended that diagnostic measurements that are taken in order to determine if radon mitigation is needed to be done close to a radon entry route with a scintillation cell detector as they are particularly sensitive to this problem. Additionally, since thoron has a 55 second half life, a sample of air drawn directly from a radon exhaust pipe, from the soil, or close to a stone foundation will have increasing decay activity until the pump for the monitor is turned off.  Then, at that time, the sample will have rapidly falling decay counts if there is thoron in the sample. However, it will have slightly increasing counts if the sample contains only radon.

Often, monitors that measure radon decay products for the purpose of determining whether or not radon remediation is needed collect those of both radon and thoron decay on the filter and the long-lived thoron decay products in the air cause working levels of the continuous monitors to read higher. However, a correction can be made by measuring the decay activity for an additional four hours after the pump had turned off at the end of the measurement period. One that had a built-in correction for subtracting the thoron decay products and listing the percentage that was collected was discontinued in the 1990s.

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