Lesson 3

How does radon enter a home?

Slide 4-2

Predicted average indoor radon levels (U.S.)

This map cannot be used to determine the radon level of a particular home. Homes with high levels of radon have been found in all three zones. All homes—regardless of their zone—should be tested.

Zone 1 (red): greater than

4 pCi/L Zone 2 (orange):

2-4 pCi/LZone 3 (yellow):

less than 2 pCi/L

Slide 4-3

Predicted average indoor radon levels (Connecticut)

Zone 1 (red): greater than

4 pCi/L Zone 2 (orange):

2-4 pCi/LZone 3 (yellow):

less than 2 pCi/L

This map cannot be used to determine the radon level of a particular home. Homes with high levels of radon have been found in all three zones. All homes—regardless of their zone—should be tested.

Slide 4-4

Requirements for radon to enter a home

1. A source of radon2. A mechanism to transport

radon from the source into the home

3. An opening or pathway into the home

Slide 4-5

Level of radon in a home

Depends on• Strength of radon sources: most

important factor• Distance of the sources from the

home• Ease of transport into the home• Ventilation in the home• Environmental factors

Slide 4-6

Uranium decay series

Uranium-238

Thorium-234

Thorium-230

Radium-226

Radon-222

Protactinium-234

Slide 4-7

Sources of radon

• Soil and rock– Uranium is present at about 0.5 to 5

parts per million (ppm) in common rocks and soil

– Uranium decays into radium– Radium decays into radon

• Groundwater• Building materials containing

uranium and radium

Slide 4-8

Radon from soil and rock

• Soil gas, including radon, seeps from the ground into the air in the home

• Usually enters through the foundation– Cracks in walls and

floors– Drains– Sump holes– Dirt floors– Construction joints– Spaces around service

pipes

Slide 4-9

Radon in groundwater

• Where underlying rocks contain high levels of uranium

• Where homes rely on groundwater from private wells or small public waterworks as the main water source

Usually a problem only in small, closed water systems

Slide 4-10

Radon in small, closed groundwater systems

• Radon does not have time to decay into harmless by-products before entering a home

• Once inside the home, radon escapes from the water into the air during normal household activities:– Showering– Washing clothes or dishes– Flushing toilets

Slide 4-11

Radon in other water

Usually not a problem where homes

• Use surface water (lakes, streams, rivers, and reservoirs)– Radon usually escapes into

the air before it reaches a home

• Use groundwater from large public systems– Water is aerated (mixed with air), which allows

radon to escape– Longer transit times allow most of remaining

radon to decay into harmless products

Slide 4-12

Radon in building materials

• Materials sometimes contain radium or uranium– Brick– Granite – Concrete products– Sheet rock– Materials contaminated with radioactive refuse (rarely

used)• Usually contribute little to indoor radon

Slide 4-13

Strength of radon sources

Soil: moderate to weak radon source

Rock: strong radon

sourceRock: strong radon source

Rock:moderate

radon source

Even homes next to each other may have different radon sources, with different strengths.

Groundwater containing radon

Slide 4-14

Radon transport mechanisms

Average contribution to radon in a home

Diffusion(1-4%)

Emanation (2-5%)

Outgassing (less than

1%)

Air pressure differences (85-90% )

Push or pull radon into a home

Slide 4-15

Radon transport mechanismsAir pressure differences

• Home creates small vacuum (negative air pressure) – Draws in soil gas,

including radon

• Vacuum caused by– Temperature differences

between outside and inside air (stack effect)

– Mechanical systems– Environmental factors

Slide 4-16

Air pressure differencesStack effect

• Heated indoor air rises and moves out of cracks and holes at the top of the house

– Creates positive air pressure at the top of the home

– Creates negative air pressure (vacuum) at the bottom

• Vacuum draws in soil gas, including radon

• Effect is greatest during coldest months, when homes are closed up

Positive pressure

Warm air

Negative pressure

Soil gas (including radon)

• Effect increases because of thermal bypasses

Slide 4-17

Air pressure differences Mechanical systems

• Heating, ventilation, and air conditioning (HVAC) systems– Air distribution

blowers– Furnaces– Boilers– Wood-burning

fireplaces– Woodstoves– Other combustion

systems

• Home exhaust systems (vent air to outside)– Clothes dryers– Exhaust fans in

bathrooms, kitchens, or attics

– Central vacuum cleaners

Slide 4-18

Air pressure differences Environmental effects

• Weather– Seasons– Rain, snow,

and frost– Wind

• Other factors that seal the soil around a home

Slide 4-19

Environmental effectsSeasons

Warm weather• Open windows• Equal pressure

indoors and outdoors– L ess radon enters

• Good ventilation dilutes radon concentration

Cold weather • Closed windows• Lower pressure

indoors– More radon enters

• Poor ventilation traps radon inside

Usually more radon enters in winter

Slide 4-20

Complex effects of good ventilation (open windows)

• Reduces vacuum effect– Generally reduces radon

entry– Dilutes radon in home

• But may also increase stack effect– Thereby increases radon

entry

When you measure radon in short-term tests, should windows be

open or closed?

Slide 4-21

Complex effects of good ventilation (open windows)

When you measure radon in short-term

tests, windows must be kept

closed

• Reduces vacuum effect– Generally reduces radon

entry– Dilutes radon in home

• But may also increase stack effect– Thereby increases radon

entry

Slide 4-22

Environmental effects Rain, snow, and frost

• Rain, snow, and frost can seal the soil– Prevent radon from

escaping from around the foundation

• Rain can force soil gas into the home

Slide 4-23

Wind

Lower pressure

down-wind

Higher pressure upwind

Don’t measure radon during high winds because results may not show typical levels.

Downwind draft effect • Changes pressure around the home• Higher pressure in soil as wind pushes

beneath soil

Environmental effectsWind

Slide 4-24

Environmental effectsFactors that seal the soil

Prevent radon escape into outdoor air

• Asphalt or concrete driveways

• Concrete patios

Slide 4-25

Radon transport mechanismsDiffusion

Diffusion = movement through materials

• Radon concentration is higher at its source (underlying soil or foundation) than in indoor air

• Radon moves from area of higher concentration to area of lower concentration Higher radon

concentration

Lower radonconcentration

Slide 4-26

Radon transport mechanismsEmanation

Emanation = emission of gas from a surface by radioactive decay

• Some rocks and other building materials contain uranium or radium

• As these elements decay, radon may be created on their surfaces

• The radon may be emitted into a room

• Emanation rate depends on– Amount of radioactive materials– Surface area of the materials

Uranium

Radium

Radon

Slide 4-27

Radon transport mechanismsOutgassing

Slide 4-28

Radon transport Pathways into the home

• Natural – Pores or empty spaces in soil– Cracks in underlying rocks– Earthen areas in basements

• Artificial– Fill below foundations– Trenches for utility lines and plumbing– Water drainage systems– Other openings in foundations

Slide 4-29

Pathway characteristics

• Ease with which air moves through the pathway

• Distance from radium (radon source)• Connections with other pathways

Slide 4-30

Radon entry and levels vary

• In space– From geographic area to geographic area– From home to home– From level to level within a home

• In time– From season to season – From day to day– From hour to hour

• With changes to the home (such as additions)

Slide 4-31

Radon levels vary

• Usually highest in lower levels of home

• Higher readings in upper levels suggest unusual radon entry factors

• Levels vary because air pressure factors vary– Temperature– Rain, snow, and

frost– Use of mechanical

exhaust systems

Because radon levels vary, testing must be done over a period of time

Slide 4-32

SummaryHow radon enters a home

Slide 4-33

Summary

• Every home should be tested for radon

• Requirements for radon to enter a home:– Radon source– Mechanism to transport radon– Pathway

Slide 4-34

Summary

• Sources– Soil and rock– Groundwater– Building materials

• Mechanisms– Air pressure

differences– Diffusion– Emanation– Outgassing

• Pathways– Natural– Artificial

• Variation in radon levels– In space– In time– With changes in the

home

Slide 4-35

Activity

For a hypothetical client, summarize how radon enters a home

• Importance of testing every home• Factors that determine the radon level in a

home• Most common sources of radon• Common mechanisms of radon entry• Pathways into the home• Variations in radon levels over space and

time