measuring radon? Notes In this lesson, we are going to talk about the units that we use to measure radon.

Transcription

1 Lesson 4: What are the units for measuring radon? Lesson overview This lesson introduces the units used to measure radon. Lesson objectives By the end of this lesson, the learners will be able to: Identify three units used to measure radon and radon decay products in homes List the EPA action levels for the three units used to measure radon and radon decay products in homes Identify four factors that may affect measurements of radon decay products In this lesson, we are going to talk about the units that we use to measure radon. See slide 4 1. First, we ll talk briefly about measuring radioactivity, or the number of radioactive decays over a given period of time. The important unit here is the picocurie, which equals decays per second 1 decay every 27 seconds 2.22 decays per minute See slide 4 2. When we measure radon gas, we consider the activity (the number of decays per minute) for a given volume of air (one liter). A liter is a little larger than a quart. So 1 pci/l = 2.22 decays/minute/liter Remember the EPA action level for radon: 4 pci/l. What does the EPA action level represent in terms of radioactive decays per time and volume? Correct answer: EPA action level = 4 pci/l = 8.88 decays/minute/liter. Lesson 4 1

2 See slide 4 3. Assume that a client spent 10 hours/day (perhaps sleeping for 8 hours and watching TV for 2 hours) in a room that had a radon level of 4 pci/l. How many decays/liter would the client be exposed to each day? Answer: 5,328 days/day/liter. See slide 4 4. This slide illustrates the average exposure to radiation in the United States. As you can see, radon accounts for more exposure than any other single source. See slide 4 5. Another method of measuring radioactivity, often used outside the United States, uses units called becquerels. A becquerel equals 1 decay per second. Since a picocurie equals one decay every 27 seconds, one becquerel also equals 27 picocuries. And since one picocurie equals decays/second, one picocurie also equals becquerels. Like picocuries, becquerels measure activity. To measure radon gas, we use activity (becquerels) for a given volume of air (in this case, a cubic meter, not a liter). A cubic meter equals 1000 liters. When we do the math, we find: One picocurie per liter of radon equals 37 becquerels per cubic meter. Again, let s consider the EPA action level for radon. How would you represent it in becquerels? Correct answer: EPA action level = 4 x 37 Bq/m 3 = 148 Bq/ m 3 See slide 4 6. So far, we ve been talking about how to measure radon gas alone. However, earlier we explained that radon decay products (RDPs) Lesson 4 2

3 also contribute to health risks. In fact, the term radon sometimes refers broadly to both radon and its decay products. In particular, we measure the alpha radiation emitted by the four short lived radon decay products: Polonium 218 Lead 214 Bismuth 214 Polonium 214 See slide 4 7. We measure the radiation from radon decay products differently. For RDPs, we generally use a unit called the working level (WL). There are several ways to define the working level. One WL is the concentration of RDPs produced from one liter of air that contains 100 pci/l of radon. One WL is the amount of short lived radon progeny that exists at a single moment if a container is kept at a constant 100 pci/l. The equivalent of the EPA action level of 4 pci/l is 0.02 WL. See slide 4 8. In summary, we can express the EPA action level for radon in three forms: 4 pci/l 148 Bq/m WL As a practical matter, the form we use most often is 4 pci/l. See slide 4 9. Are there any questions so far? See slide Several factors can raise or lower the concentration of radon and RDPs that can be measured. Some radon gas and RDPs naturally escape as air flows out of the home. Lesson 4 3

4 As we noted earlier, unlike radon gas, radon decay products are Solid particles Electrically charged Chemically reactive These characteristics determine how RDPs behave and how they are measured. Some RDPs attach to (or plate out on) solid objects, such as walls, floors, ceilings, and furniture. Plating out lowers their concentration in the air. Plated out RDPs cannot be measured. o Only the RDPs that remain in the air can be measured. o Therefore, factors that affect plating out also affect the measurement of RDPs. See slide Among the factors that we need to consider are Air circulation Ventilation Air filters Particles suspended in the air. See slide Let s first consider air circulation (that is, moving around the air that is already within a room). For example, fans increase normal air circulation. How do you think air circulation would affect the concentration of RDPs? Correct answer: Circulation, or moving air, may increase plating out, as RDP particles blow toward solid objects and attach to them. When plating out increases, the concentration of RDPs in the air decreases. See slide Lesson 4 4

5 We said that circulation refers to moving around air that is already in the space. In contrast, ventilation refers to a supply of fresh air. Open windows and doors increase ventilation. What effect do you think ventilation would have on the concentration of RDPs? Correct answer: Again, ventilation is likely to lower the concentration of RDPs. It may also reduce the concentration of radon, as the gas escapes from the home. See slide Another factor that affects the concentration of RDPs is an air filter in a furnace, for example. How do you think an air filter would affect the concentration of RDPs? Correct answer: A filter might remove some RDPs, which are charged particles that are chemically active. A filter would not remove radon gas itself, which has no electrical charge and is not chemically active. Thus, a filter would lower the concentration of RDPs. See slide What effect do you think particles suspended in the air (such as dust, smoke, and aerosols) might have? Correct answer: When there are particles in the air, RDPs are more likely to attach to these particles. Thus the RDP concentration in the air is likely to decrease. Lesson 4 5

6 See slide As you see, air circulation, ventilation, air filters, and particles in the air can all affect radon measurement. We ll see the importance of these factors again when we discuss the conditions that are needed to measure radon during short term tests. See slide Earlier, when we described radon, we said that over time, radon decays into various short lived radon decay products. In a closed system, like a closed home, the concentration of RDPs in the house increases until a situation called secular equilibrium is reached. (In this term, secular means eventual.) Secular equilibrium occurs when the radon decay products have the same level of radioactivity as the radon itself. In other words, the rate of decay of each short lived RDP will equal the rate of decay of the radon itself. Achieving secular equilibrium takes about 3 to 4 hours. You will need to measure radon in a home after it has reached secular equilibrium. When we talk about the necessary conditions for shortterm testing, you ll see that these conditions reflect the time needed to achieve secular equilibrium. See slide This slide is intended as a little light relief and as a reminder of why the information in this section is important. See handout 4 1. Handout 4 1 summarizes this section on the units we use to measure radon and its decay products. Summary. See slide Let s review the units for measuring radon and radon decay products: Picocuries/liter Becquerels/cubic meter Working levels Lesson 4 6

7 What are the EPA action levels for these methods of measurement? Correct numbers: 4 pci/l 148 Bq/m WL See slide We also talked about factors that affect the measurement of radon and radon decay products, including Air circulation Ventilation Air filters Particles in the air These factors are important when we consider the conditions necessary for testing a home. See slide Do you have any questions about the units for measuring radon? See slide Check comprehension. See handout 4 2A. This comprehension check is not graded. Now you re going to see whether you remember the main points that we ve discussed in this lesson. Please answer the questions on handout 4 2A. When you all finish, we ll review the answers together. Review the answers. See handout 4 2B, the answer key. Lesson 4 7

8 Resources U.S. Environmental Protection Agency Radon Proficiency Program (RPP) Handbook. Residential Measurement and Mitigation Proficiency. EPA 402 R July. Accessed January 23, EPA Assessment of risks from radon in homes United States Air and Radiation (6608J). EPA 402 R June. Lesson 4 8

9 Handout 4 1: Units for measuring radon and its decay products Radon gas U.S. system Measuring activity (number of radioactive decays per unit of time) 1 picocurie (pci) = one trillionth of curie (1Ci x ) = decays/second (dps) = Bq = 2.22 decays/minute (dpm) = 1 decay/27 seconds Measuring radon (number of decays per unit of time per volume) 1 picocurie/liter of air (pci/l) = 2.22 decays/minute/liter EPA action level 4 pci/l International system Measuring activity (number of radioactive decays per unit of time) 1 becquerel (Bq) = 1 decay/second = 27 pci Measuring radon (number of decays per unit of time per volume) 1 becquerel/cubic meter of air (Bq/m 3 ) = 1 decay/second/m 3 Working level (WL) Radon decay products Measuring concentration of radon decay products in volume of air = Any combination of short lived radon decay products that will produce 1.3 x 10 5 million electron volts of alpha energy per liter of air Lesson 4 9

10 Conversions If you have picocuries/ liter (pci/l) And you want becquerels/ cubic meter * (Bq/m 3 ) If you have becquerels/ cubic meter * (Bq/m 3 ) And you want picocuries/ liter (pci/l) Method Multiply pci/l by 37 Multiply Bq/m 3 by Example: EPA action level 4 pci/l x 37 = 148 Bq/m Bq/m 3 x = 4 pci/l Equation 1 pci/l = 37 Bq/m 3 1 Bq/m 3 = pci/l Working levels Method Example: EPA action level Equation If you have picocuries/ liter (pci/l) And you want working levels (WL) Multiply pci/l by the equilibrium ratio (ER) (usually assumed to be 0.5) and divide by 100 WL = 4 x 0.5 = WL = pci/l x ER 100 If you have working levels (WL) And you want picocuries/ liter (pci/l) Multiply WL by 100 and divide by equilibrium ratio (ER) (usually assumed to be 0.5) pci/l = 0.02 x 100 = pci/l = WL x 100 ER * 1 cubic meter (1 m 3 ) = 1000 liters (1000 L) Lesson 4 10

11 Handout 4 2A: Check your understanding Select the best answer from the choices below. Circle the correct answer. 1. Radon activity means a. How many radon atoms enter a home within a given period of time b. How many radon atoms spread through a home within a given period of time c. The number of radioactive decays in a given period of time for a given volume of air d. The number of radon atoms moving within a given volume of air 2. We usually measure radon activity in a. Curies/cubic meter of air b. Picocuries/liter of air c. Picocuries/cubic meter of air d. Liters of radon gas/picocurie 3. Using the units above (from question 2), the EPA action level for radon is a b. 0.4 c. 4.0 d In units used in the International System, the EPA action level for radon is a. 37 Becquerels/cubic meter b. 148 Becquerels/cubic meter c. 73 mackerels/liter d. 148 working levels/cubic meter 5. The units we use to measure radon decay products are a. Microcuries/liter of air and acting levels b. Picocuries/liter of air and working levels c. Secular equilibrium ratios d. Functional levels 6. The length of time to achieve secular equilibrium is about a. 7 days b. 3 7 minutes c. 3 4 days d. 3 4 hours 7. Which of the following factors probably will not affect radon measurement a. Open windows b. Attic fans c. Dust in the air d. Furniture placement Lesson 4 11

12 Handout 4 2B: Check your understanding Answer key The correct answers are shown in bold. 1. Radon activity means a. How many radon atoms enter a home within a given period of time b. How many radon atoms spread through a home within a given period of time c. The number of radioactive decays in a given period of time for a given volume of air d. The number of radon atoms moving within a given volume of air 2. We usually measure radon activity in a. Curies/cubic meter of air b. Picocuries/liter of air c. Picocuries/cubic meter of air d. Liters of radon gas/picocurie 3. Using the units above (from question 2), the EPA action level for radon is a b. 0.4 c. 4.0 d The units we use to measure radon decay products are a. Microcuries/liter of air and acting levels b. Picocuries/liter of air and working levels c. Secular equilibrium ratios d. Functional levels 6. The length of time to achieve secular equilibrium is about a. 7 days b. 3 7 minutes c. 3 4 days d. 3 4 hours 7. Which of the following factors probably will not affect radon measurement a. Open windows b. Attic fans c. Dust in the air d. Furniture placement 4. In units used in the International system, the EPA action level for radon is a. 37 Becquerels/cubic meter b. 148 Becquerels/cubic meter c. 73 mackerels/liter of water d. 148 working levels/cubic meter Lesson 4 12