Lesson 3
How does radon
enter a home?
Predicted average indoor
radon levels (U.S.)
Zone 1 (red):
greater than
4 pCi/L
Zone 2 (orange):
2-4 pCi/L
Zone 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.
Slide 3-1
All homes—regardless of their zone—should be tested.
Predicted average indoor
radon levels (Connecticut)
Zone 1 (red):
greater than
4 pCi/L
Zone 2 (orange):
2-4 pCi/L
Zone 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.
Slide 3-2
All homes—regardless of their zone—should be tested.
Requirements for radon
to enter a home
1. A source of radon
2. A mechanism to transport radon
from the source into the home
3. An opening or pathway into the
home
Slide 3-3
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 3-4
Where radon
comes from
Uranium-238
Thorium-234
See handout 3-1
Protactinium-234
Thorium-230
Radium-226
Radon-222
Slide 3-5
Sources of radon
• Soil and rock: most common source
• Groundwater
• Building materials containing
uranium and radium
Slide 3-6
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 3-7
Overview
Radon from soil and rock
• Radon enters soil gas
• Soil gas moves from
ground into air in home
• Usually enters through
foundation
– Cracks in walls and
floors
– Drains
– Sump holes
– Dirt floors
– Construction joints
– Spaces around service pipes
Slide 3-8
Radon source
Groundwater
Usually a problem only in
small, closed water
systems
• 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
Slide 3-9
Radon in small, closed
groundwater systems
• Radon does not have time to
decay into harmless byproducts 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 3-10
Radon in other water
Usually not a problem where homes
• 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
Slide 3-11
Radon source
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 3-12
Strength of
radon sources
Even homes next to each other may have
different radon sources, with different strengths.
Rock:
strong radon
source
Groundwater
containing radon
Rock:
strong
radon
source
Rock:
moderate
radon source
Soil: moderate to weak radon source
Slide 3-13
Questions?
• About radon sources
• Next
– Mechanisms that transport radon into a
home
Slide 3-14
Radon
transport mechanisms
Push or pull radon into a home
Average contribution to
radon in a home
Diffusion
(1-4%)
Emanation
(2-5%)
Outgassing
(less than
1%)
Air pressure
differences
(85-90%)
Slide 3-15
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 3-16
Air pressure differences
Stack 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 3-17
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 3-18
Air pressure differences
Environmental effects
• Weather
–Seasons
–Rain, snow,
and frost
–Wind
• Other factors
that seal the
soil around a
home
Slide 3-19
Environmental effects
Seasons
Usually more radon enters in winter
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
Slide 3-20
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 3-21
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,
windows must
be kept closed
Slide 3-22
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 3-23
Environmental effects
Wind
Don’t measure
radon during
high winds
because
Higher
results may
pressure
upwind
not show
typical levels.
Downwind draft effect
• Changes pressure around the home
• Higher pressure in soil as wind pushes
beneath soil
Lower
pressure
downwind
Slide 3-24
Environmental effects
Factors that seal the soil
Prevent radon from
escaping into
outdoor air
• Asphalt or
concrete
driveways
• Concrete
patios
Slide 3-25
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 3-26
Other radon transport mechanisms
Diffusion
Diffusion = movement through materials
Lower radon
concentration


Higher radon
 concentration 
• 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
Slide 3-27
Other radon transport mechanisms
Emanation
•
•
•
•
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
Radon
room
Radon
↑
Emanation rate depends on
Radium
– Amount of radioactive materials
– Surface area of the materials
↑
Uranium
Slide 3-28
Other radon transport mechanisms
Outgassing
Outgassing = release of radon gas from water
Slide 3-29
Questions?
• About mechanisms that transport
radon into a home
• Next
– Pathways that allow radon to enter a
home
Slide 3-30
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 3-31
Pathway characteristics
• Ease with which air moves through
the pathway
• Distance from radium (radon source)
• Connections with other pathways
Slide 3-32
Review
Requirements for radon entry
1. Radon source
2. Mechanism to transport radon from
source into home
3. Pathways into the home
These factors determine the amount of
radon that enters a home.
Slide 3-33
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 3-34
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)
• Factors
– Air pressure changes
– Wind speed and
direction
– Indoor and outdoor
temperature changes
– Rain, snow, and frost
– Use of mechanical
exhaust systems
Because radon levels vary, testing
must be done over a period of time
Slide 3-35
Why this information
matters
Procedures for measuring radon are
designed to control for these factors:
• Air pressure differences
• Ventilation
• Environmental effects
Slide 3-36
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 3-37
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 3-38
Summary
How radon enters a home
Slide 3-39
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
Hypothetical client:
• Variations in radon levels
an attorney
over space and time
Slide 3-40
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 3-41
Questions
• About how radon enters a home
Slide 3-42
Check
your understanding
• See handout 3-4
Slide 3-43