Lesson 3

How does radon enter a home?

Slide 3-2

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): Zone 1 (red): greater than greater than

4 pCi/L 4 pCi/L Zone 2 (orange): Zone 2 (orange):

2-4 pCi/L2-4 pCi/LZone 3 (yellow): Zone 3 (yellow):

less than less than 2 pCi/L2 pCi/L

Slide 3-3

Predicted average indoor radon levels (Connecticut)

Zone 1 (red): Zone 1 (red): greater than greater than

4 pCi/L 4 pCi/L Zone 2 (orange): Zone 2 (orange):

2-4 pCi/L2-4 pCi/LZone 3 (yellow): Zone 3 (yellow):

less than less than 2 pCi/L2 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 3-4

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 3-5

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 3-6

Where radon comes from

Uranium-238

Thorium-234

Thorium-230

Radium-226

Radon-222

Protactinium-234See handout 3-1

Slide 3-7

Sources of radon

• Soil and rock: most common source• Groundwater• Building materials containing

uranium and radium

Slide 3-8

Radon source Soil and rock

• Uranium is present at about 0.5 to 5 parts per million (ppm) in common rocks and soil– Uranium and radium especially

common in granites, shales, and limestones

– Under a home, they can be source of radon

• Uranium decays into radium– Radium decays into radon

Slide 3-9

Overview Radon from soil and rock

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

• Radon enters soil gas• Soil gas moves from

ground into air in home• Usually enters through

foundation– Cracks in walls and

floors

Slide 3-10

Radon source 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 3-11

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 3-12

Radon in other water

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

air before it reaches a home

• Use groundwater from large public systems– Water is aerated (mixed with air) and escapes– Longer transit times allow most of remaining

radon to decay into harmless products

Usually not a problem where homes

Slide 3-13

Radon sourceBuilding 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 3-14

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 3-15

Questions?

• About radon sources

• Next– Mechanisms that transport radon into a

home

Slide 3-16

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 3-17

Main radon transport mechanism Air 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 3-18

Air pressure differencesStack effect

• Heated indoor air rises and escapes through cracks and holes at top of home– Creates positive air

pressure at top of home– Creates negative air

pressure (vacuum) at bottom

• Vacuum draws in soil gas, including radon

• Effect is greatest during coldest months

• Thermal bypasses increase effect

Positive pressure

Warm air

Negative pressure

Soil gas (including radon)

Slide 3-19

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 3-20

Air pressure differences Environmental effects

• Weather– Seasons– Rain, snow,

and frost– Wind

• Other factors that seal the soil around a home

Slide 3-21

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 3-22

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 3-23

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 3-24

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 3-25

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 3-26

Environmental effectsFactors that seal the soil

• Asphalt or concrete driveways

• Concrete patios

Prevent radon from escaping into outdoor air

Slide 3-27

Summary Air pressure

• Main mechanism that brings radon into a home– Difference in air pressure between indoor and

outdoor air

• Main causes of air pressure differences– Temperature differences between indoor and

outdoor air– Mechanical systems– Environmental factors

Slide 3-28

Other radon transport mechanisms

Diffusion

• 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

Diffusion = movement through materials

Slide 3-29

Radon

Other radon transport mechanisms Emanation

• Some rocks and other building materials contain uranium or radium

Radon↑

Radium↑

Uranium

Emanation = emission of gas from a surface by radioactive decay

• 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

Slide 3-30

Other radon transport mechanisms Outgassing

Outgassing = release of radon gas from water

Slide 3-31

Questions?

• About mechanisms that transport radon into a home

• Next– Pathways that allow radon to enter a

home

Slide 3-32

Radon 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 3-33

Pathway characteristics

• Ease with which air moves through the pathway

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

Slide 3-34

ReviewRequirements for radon entry

1. Radon source2. Mechanism to transport radon from

source into home3. Pathways into the home

These factors determine the amount of radon that enters a home.

Slide 3-35

Radon levels vary in space

• In space– From geographic area to geographic

area– From home to home– From level to level within a home

• Usually highest in lower levels of home• Higher readings in upper levels suggest

unusual radon entry factors

Slide 3-36

Radon levels vary over time

• In time– From season to

season – From day to day– From hour to hour

• With changes to the home (such as additions)

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

• Factors – Air pressure changes– Wind speed and

direction– Indoor and outdoor

temperature changes– Rain, snow, and frost– Use of mechanical

exhaust systems

Slide 3-37

Why this informationmatters

• Air pressure differences• Ventilation• Environmental effects

Procedures for measuring radon are designed to control for these factors:

Slide 3-38

Summary

• Every home should be tested for radon

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

See handout 3-3

Slide 3-39

Summary

• Sources– Soil and rock– Groundwater– Building materials

• Mechanisms– Air pressure

differences– Diffusion– Emanation– Outgassing

• Pathways– Distance from radon

source– Ease with which air moves– Connections with other

pathways

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

Slide 3-40

SummaryHow radon enters a home

Slide 3-41

Activity

• 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

Hypothetical client: an attorney

For a hypothetical client, summarize how radon enters a home

Slide 3-42

Activity review

• Is the explanation complete?• Is the explanation clear?• Does the explanation contain the

right level of technical detail for this client?

See handout 3-2 for a sample explanation

Slide 3-43

Questions

• About how radon enters a home

Slide 3-44

Check your understanding

• See handout 3-4