Transcript 2b. Indoor Air Quali..

The Indoor Environment
GISAT 112
Overview of the Indoor Environment
• Amount of time spent indoors…
• Health and safety risks: accidents, food,
water, air…
• Emphasis here on indoor air quality*
– Pollutants, sources, effects
– Effects of indoor-outdoor air exchange
*but
not in the workplace
Activity Patterns
Source: EPA, Exposure Factors Handbook, August 1997
Indoor activities
Adults (12 and older)
21 h/day
Children (3-11)
19 h/day (weekdays)
17 h/day (weekends)
Outdoor activities
Adults
1.5 h/day
Children
5 h/day (weekdays)
7 h/day (weekends)
Time spent inside a vehicle (adults) 1.3 h/day
Overall (adults):
Indoor ~90%
Outdoor ~10%
Pollutants
• Gases
– Inorganic: CO, NOx, SOx, radon….
– Organic: formaldehyde, pesticides, “VOCs”….
• Particles
– Inorganic: combustion aerosols, “dust”….
– Organic: spray aerosols, cooking smoke, ETS…
– Biogenic: bacteria, fungi, spores, dander, antigens….
National Emissions vs. Exposures
Example: Benzene
Health Risk
RISK = TOXICITY x EXPOSURE
• Where toxicity is a measure of adverse
health effect per unit mass of pollutant
• And exposure is a measure of the mass of
pollutant breathed over a period of time
Volume of Air Breathed
• Typically, about
15 - 20 m3/day
– i.e., the volume of a small bedroom
• More for athletes in action
• Less for couch potatoes and children
• About 20 kg per day
• For comparison: food and water- ~2 kg/day each
Effects
• Odor
• Irritation of mucous membranes
• Lung damage
• Infection
• Asthma
• Cancer
• Other systemic effects
Worker Productivity
Radon in the home
•
•
•
•
What is Radon?
What are the properties of Radon?
Where does it come from?
How does it get into the house?
What is Radon?
• Radon (chemical symbol Rn) is a naturally
occurring radioactive gas found in soils,
rock, and water throughout the U.S.
• Radon causes lung cancer, and is a threat to
health because it tends to collect in homes,
sometimes to very high concentrations.
• It has numerous different isotopes, but
radon-220, and -222 are the most common.
Source: US EPA
Isotopes
• A given element might exist in different
forms – “isotopes” – with differing numbers
of neutrons
• If they have the same number of protons,
they are the same element
• But their weight is different because of the
neutrons
• Example: Hydrogen, Deuterium, Tritium
E.g.: Carbon 14
What are the properties of radon?
• Radon is a noble gas, which means it is essentially inert,
and does not combine with other chemicals.
• Radon is a heavy gas, which accounts for its tendency to
collect in basements.
• It has no color, odor, or taste.
• Radon-222 is produced by the decay of radium, has a halflife of 3.8 days, and emits an alpha particle as it decays to
polonium-218, and eventually to stable lead.
• Radon-220, is the decay product of thorium – it is
sometimes called thoron, has a half-life of 54.5 seconds
and emits an alpha particle in its decay to polonium-216.
Source: US EPA
Where does Rn come from?
Radon Precursors and Progeny
(Masters, Figure 7.54)
How does Radon get into the house?
• Radon is “pumped”
into your house by the
heating system
– Inhalation exposure
• A smaller source can
be well water pumped
into your house
– Inhalation and
ingestion exposure
Source: US EPA
Radon Risk if You Smoke
Radon
Level
If 1,000 people who smoked were exposed to this level
over a lifetime...
The risk of cancer from radon exposure
compares to...
WHAT TO DO:
Stop smoking and...
20 pCi/L
About 135 people could get lung cancer
100 times the risk of drowning
Fix your home
10 pCi/L
About 71 people could get lung cancer
100 times the risk of dying in a home fire
Fix your home
8 pCi/L
About 57 people could get lung cancer
4 pCi/L
About 29 people could get lung cancer
100 times the risk of dying in an airplane
crash
Fix your home
2 pCi/L
About 15 people could get lung cancer
2 times the risk of dying in a car crash
Consider fixing between 2 and 4
pCi/L
1.3 pCi/L
About 9 people could get lung cancer
(Average indoor radon level)
(Reducing radon levels below 2
pCi/L is difficult.)
0.4 pCi/L
About 3 people could get lung cancer
(Average outdoor radon level)
(Reducing radon levels below 2
pCi/L is difficult.)
Fix your home
Note: If you are a former smoker, your risk may be lower.
Source: www.epa.gov/iaq/radon/riskcht.html
Radon Risk if You Never Smoked
Radon
Level
If 1,000 people who never smoked were exposed to this
level over a lifetime...
The risk of cancer from radon exposure
compares to...
WHAT TO DO:
20 pCi/L
About 8 people could get lung cancer
The risk of being killed in a violent crime
Fix your home
10 pCi/L
About 4 people could get lung cancer
8 pCi/L
About 3 people could get lung cancer
10 times the risk of dying in an airplane
crash
Fix your home
4 pCi/L
About 2 people could get lung cancer
The risk of drowning
Fix your home
2 pCi/L
About 1 person could get lung cancer
The risk of dying in a home fire
Consider fixing between 2 and 4
pCi/L
1.3
pCi/L
Less than 1 person could get lung cancer
(Average indoor radon level)
(Reducing radon levels below 2
pCi/L is difficult.)
0.4
pCi/L
Less than 1 person could get lung cancer
(Average outdoor radon level)
(Reducing radon levels below 2
pCi/L is difficult.)
Fix your home
Note: If you are a former smoker, your risk may be higher.
Source: www.epa.gov/iaq/radon/riskcht.html
Indoor Air Pollutants
(from Miller, Env.Sci., 9th ed., and EPA)
Indoor Biocontaminants
• The previous diagram leaves out a major class of
indoor pollutants, sometimes generally referred to
as “biocontaminants”.
• Examples: fungi, bacteria, viruses, spores, animal
dander, pollen, insect fragments, dust mites.
• Microbial contaminants thrive in or on moist
materials.
• Control by preventing leaks and high-moisture
areas; cleaning regularly; and air cleaning if
necessary.
There’s Money in Mold
(from the Raleigh News and Observer, 12-20-01)
Regulations
• Outdoor Air
– SOx, NOx, CO, Ozone, Particles, Pb
– Hazardous air pollutants (189/33)
• Indoor Air
–
–
–
–
–
–
Codes: ventilation, combustion devices
Asbestos removal
Product labeling (e.g., carpets)
Toxic substance warnings (e.g., in CA)
Smoking bans
Radon (guidelines for acceptable levels; testing
required by some States when houses sold)
Ventilation Terminology
• Natural ventilation
– Infiltration and exfiltration occur by natural
driving forces (temperature differences, wind)
• Mechanical ventilation
– Air forced into or exhausted from buildings by
fans
– Dilution ventilation: dilute indoor-generated
pollutants below thresholds
– Exhaust ventilation: extract high-concentration
pollutants or moisture
Ventilation Rates
• Ventilation rates are often expressed as air
exchange rates, in air changes per hour (ACH)
air flow rate ÷ room volume = ACH [hr-1]
• For example, a 300 m3 house through which
150 m3/hr of air is entering via infiltration
(and exiting via exfiltration) is experiencing
an ACH of 0.5 hr-1 due to natural ventilation
How Air Pollutants Move
• By (molecular) diffusion
– Kinetic activity
– Relevant for gases, very fine particles
• By convection
– Pollutants carried by air that moves by temperature
differences (warm air rises…)
– Pollutants carried by air that is mechanically driven, or
wind-driven
• By settling
– Force of gravity pulls airborne particles downward
– Relevant for particles larger than about 1 μm diameter
Volatility and Evaporation
• Some liquid materials (and a few solid ones) are
relatively volatile.
• This means that at about room temperature, these
materials—or some of their constituents—tend to
evaporate.
• During evaporation, molecules escape from a
liquid and become gaseous (vapors).
• Volatile organic compounds (VOCs) are
particularly common air contaminants, and many
synthetic materials inside buildings emit them.
Concentrations Can Be Highly Variable
Mass (or Material) Balances
• Consider the RATE of material flow
• Draw a boundary around system
– e.g. a bathroom sink
• In the simplest form:
Input – Output = Accumulation
• More generally:
Input – Output + Generation – Consumption = Accumulation
• Or:
I–O+G–C=A
Mass Balance Example
• How many ACH are needed to keep the concentration of
formaldehyde at or below 10 μg/m3 in a 2.5 x 4 x 30 m
room, if there is a constant source of 50 μg/h in the room?
(Assume complete mixing and steady-state.)
• So for I – O + G – C = A:
I = 0 μg/h (assuming clean air coming in)
This is the “volumetric
flow rate” of the system
O = (10 μg/m3)(Volume*ACH) = (10 μg/m3)(300 m3)(ACH)
G = 50 μg/h
C = 0 μg/h (assuming nothing in the room eats formaldehyde!)
A = 0 μg/h (assuming steady state)
0
0
0
• So I – O + G – C = A simplifies to
O=G
Mass Balance Example (cont’d)
O=G
(10 μg/m3)(300 m3)(ACH) = 50 μg/h
50 g /h
ACH 
(10 g/m 3 )(300 m3 )
=
1.7 x 10-2 /h
Air Exchange and Ventilation
• Ventilation: exchange of indoor and outdoor air
• Diagram below is from an IAQ modeling tool.
Values represent airflows between rooms and
between indoors and outdoors.
• Let’s look at a simple mass balance around a
room…
Mass Balance Example
• A room with a volume of 25 m3 contains a source of pollutant
“P” that emits P at 120 micrograms per hour.
• Air flows into this room (from adjacent rooms and/or the
outdoors) at 20 m3 per hour. Air flows out at the same rate.
• The air flowing into the room contains a small amount of P…10
μg/m3.
• Assume that the source has been in the room long enough to
bring the concentration of P up to a steady value.
• Assume there are no reactions that destroy P and no surfaces in
the room that adsorb it.
• Run a mass balance, then calculate the concentration of P in the
room.
Mass Balance for a Pollutant
Room volume = 25 m3
20 m3/h
Pin = 10 μg/m3
20 m3/h
Poll. Gen = 120 μg/h
Pout = ? μg/m3
Output = Input + Generation – Consumption – Accumulation
So 20•Pout = 20•Pin + 120 – 0 – 0 = 200 + 120 = 320 μg/h
Therefore, under steady conditions 320 μg/h of P is flowing out of the
room.
The concentration coming out is 320 μg/h ÷ 20 m3/h = 16 μg/m3.
If the air in the room is well-mixed, that is also the concentration in the
room.