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Why Measure RDPs?

 

 

Measurement of Radon Decay Products
Although one routinely measures radon and compares the results to a guideline of 4.0 pCi/L, it is important to note that the actual health risk comes from the short-lived decay products of radon. As a gas, radon enters a building due to pressure differentials. Once in a building, the radon will diffuse throughout the structure where it can be inhaled. However, as an inert gas, it is inhaled and exhaled without an accumulation in the lungs and is therefore not a health risk in itself.

Radon is an unstable element and will radioactively decay to other elements that behave as solids. These decay products of radon have electrostatic charges, which as they are inhaled, cause them to adhere to the lung tissue. While attached to the sensitive lung tissue, they can damage the exposed, unshielded cells as they continue to decay and release alpha radiation. This occurs rapidly and well before the lungs have an opportunity to expel them through the natural clearing process.

The measurement of radon decay products was the primary methodology used during the 1960s to mid 1980s as a means to monitor employee exposures in underground mines. Radon decay product measurements are expressed in units of Working Levels (WL) and exposure in terms of a time weighted average referred to as Working Level Months (WLM). The correlation of actual exposure to radon decay products (WLM) to the incidence of lung cancer is the basis of the health studies from uranium miner exposures that form the foundation of EPAs recommendations and OSHA regulations.

Radon decay product measurements were also the basis for clean-up efforts in certain western US states where uranium mill tailings had inappropriately been used for building materials. In these cases, a residential clean up standard of 0.02 WL of radon decay products was established. In determining compliance of these clean-up efforts, fairly intricate measurement equipment was used that actively took air samples and measured the activity of the particulates collected from the air. At that point in time, research was begun to identify measurement methods that could more easily estimate radon decay product concentrations by measuring the radon that produced them.

The development of inexpensive measurement methods became even more important when concerns of radon exposure in residential structures arose during the mid-1980s. The need to measure a large number of homes accelerated the development of short-term radon measurement devices that could estimate radon decay product exposures. These included activated charcoal and electret ion chambers. To develop these devices, an assumption was necessary to allow one to calculate the amount of radon required to cause levels of radon decay products to be at 0.02 WL. This assumption is referred to as the equilibrium factor, or equilibrium ratio.

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Equilibrium Factor
The equilibrium factor is a simple mathematical formula (see below for formula) that estimates, for a given radon level in the air space of a room, how many of the decay products produced are actually in the air space and available for inhalation. Since the radon decay products have a strong electrostatic charge, they have a tendency to adhere to other objects, in the same manner as they can adhere to lung tissue. These radon decay products will adhere to walls, furniture, windows etc. when they come in contact with them. Once they contact these objects, the electrostatic forces are strong enough that air movement, dusting, or even physical contact cannot dislodge them, and hence, no longer represent a respiration health risk. Furthermore, the alpha particle released from these attached decay products travel approximately an inch through the air before their energy is absorbed, and do not present a risk to occupants. Even if they did strike the skin, the skin provides a sufficient dead cell barrier to shield live cells from this improbable contact. Consequently, the equilibrium fraction expresses the distribution between those radon decay products that are attached to physical objects (not a hazard) versus those that are still airborne (present a hazard).

The attachment of radon decay products to physical objects in a room is significantly impacted by air circulation (increased air movement increases the probability of contact and attachment to objects), as well as the presence of air filtration devices. The preferential attachment of the electrostatically charged radon decay products to air borne dust particles (such as cigarette smoke) increases the amount of radon decay products in the air.

Conversely, devices that remove airborne dust particles (media filters) also reduce radon decay products that are attached to them.

The physical factors that affect the fraction of decay products available for inhalation can cause a wide range of differences from one building to the next when radon measurement devices are used to estimate actual radon decay product exposure. However, to meet the demand for inexpensive radon measurement devices to estimate residential exposure, an equilibrium factor of 50% was assumed. The US EPA has used this assumption in developing their radon program. Simply stated, the 50% equilibrium factor assumption means that for a given amount of radon, it is assumed that half of the decay products produced are plated out (non-hazardous) and the other half are available for inhalation (hazardous). This assumption allowed the development of an equivalent guideline in terms of the more easily measured radon level as follows:

Rn = 100 x 0.02WL/.50 = 4.0 pCi/L

It is important to note that actual equilibrium factors can vary significantly from one building to the next. It has been the experience of researchers that in homes and commercial buildings with forced air heating or cooling systems; the actual factors can often be below 20%. In situations where forced air systems are routinely operated, the radon levels that would cause the radon decay products to be in excess of 0.02WL could be much higher. For example in a situation where the F was determined to be 20%, the equivalent radon would be:

Rn = 100x .02WL/.20 = 10 pCi/L

This example illustrates the large difference of radon measurements that can represent the same actual risk in terms of radon decay products. In situations where radon mitigation is relatively inexpensive, it may be prudent to proceed with mitigation based on the surrogate measurement of radon and the assumed 50% equilibrium factor. However, in situations where high air recirculation rates are common within a building (such as in schools, commercial buildings and homes where forced air systems operate most of the time) and when radon mitigation is expensive, performing a follow-up test that measures the radon decay products directly may be advisable before proceeding to mitigation.

Equilibrium Factor Formula
The equilibrium factor formula is expressed as follows: F = 100xWL/Rn where WL = radon decay product measurement in WL and Rn = radon activity in pCi/L and F is a decimal fraction. By assuming an F one can estimate radon decay products from a radon measurement or conversely radon from a radon decay product measurement.

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Last modified: 06/05/08