Indoor Air Pollution - Air Pollution and Global Warming: History, Science, and Solutions - page 209

outdoors. The indoor-to-outdoor ratio of ozone ranges
from 0.1:1 to 1:1, with typical values of 0.3:1 to 0.5:1
(Finlayson-Pitts and Pitts, 1999). Ozone is lost indoors
by reaction with wall, floor, and ceiling surfaces; reac-
tion with indoor gases; and deposition to floors. Health
effects of ozone are discussed in Section 3.6.5.

larly in their basements, overlying soils with uranium-
rich rocks (Nazaroff and Nero, 1988). Radon plays no
role in acid deposition, stratospheric ozone, or outdoor
air pollution problems. Because of its indoor effects,
radon is considered a hazardous pollutant in the United
States under the Clean Air Act Amendments of 1990
(CAAA90).

9.1.5. Sulfur Dioxide
Sulfur dioxide [SO 2 (g)], emitted outdoors during gaso-
line, diesel, and coal combustion, is emitted indoors
during combustion of kerosene for space heaters or
wood for heating and cooking. In the absence of indoor
sources, indoor SO 2 (g) mixing ratios are typically
10 to 60 percent those of outdoor air (Jones, 1999).
Once indoors, SO 2 (g) does not chemically degrade
quickly in the gas phase because the hydroxyl radi-
cal [OH(g)] required to initiate its breakdown is not
produced indoors. Because the e -folding lifetime of
the hydroxyl radical is about 1 second, OH(g) brought
indoors from the outside disappears quickly. Losses
of SO 2 (g) include deposition to wall and floor sur-
faces, dissolution into liquid water (e.g., in bathtubs
and sinks), and dissolution into aerosol particles con-
taining liquid water. The health effects of sulfur dioxide
are discussed in Section 3.6.6.

9.1.7.1. Sources and Sinks
The ultimate source of radon gas is the radioactive decay
of solid mineral uranium-238 ( 238 U), where the 238
refers to the isotope, or number of protons plus neu-
trons in the nucleus of a uranium atom. Of all ura-
nium on Earth, 99.2745 percent is 238 U, 0.72 percent is
235 U, and 0.0055 percent is 234 U. 238 U has a half-life of
4.5 billion years.
Radon formation from uranium involves a long
sequence of radioactive decay processes. During radio-
active decay of an element, the element spontaneously
emits radiation in the form of an alpha ( ) particle, beta

( )ray. An alpha particle is

the nucleus of a helium atom, which is made of two
neutrons and two protons (Figure 1.1). It is the least
penetrating form of radiation and can be stopped by a
thick piece of paper. Alpha particles are not dangerous
unless the emitting substance is inhaled or ingested. A
beta particle is a high-velocity electron. Beta particles
penetrate deeper than do alpha particles, but less than
do other forms of radiation, such as gamma rays. A
gamma ray is a highly energized, deeply penetrating
photon emitted from the nucleus of an atom not only
during nuclear fusion (e.g., in the sun's core), but also
sometimes during radioactive decay of an element.
The French physicist Antoine Henri Becquerel
(1871-1937; Figure 9.1) discovered radioactive decay
on March 1, 1896. To obtain his discovery, Becquerel
placed a uranium-containing mineral on top of a pho-
tographic plate wrapped by thin, black paper. After
letting the experiment sit in a drawer for a few days,
he developed the plate and found that it had become
fogged by emissions (Figure 9.2) that he traced to the
uranium in the mineral. He referred to the emissions
as metallic phosphorescence .What he had discov-
ered was the emission of some type of particle due
to radioactive decay. He repeated the experiment by
placing coins under the paper and found that their out-
lines were traced by the emissions. Two years later, the
New Zealand-born, British physicist Ernest Ruther-
ford (1871-1937; Figure 9.3) found that uranium emit-
ted two types of particles, which he named alpha and

) particle, or gamma (

9.1.6. Formaldehyde
Formaldehyde [HCHO(g)], produced during fossil
fuel and solid and liquid biofuel burning and chemical
reaction outdoors, is emitted from particleboard, insu-
lation, furnishings, paneling, plywood, carpets, ceiling
tile, tobacco smoke, and combustion indoors. Formalde-
hyde mixing ratios indoors are usually greater than are
those outdoors. In outdoor air, formaldehyde breaks
down by photolysis and reactions with HO 2 (g) and
OH(g). UV sunlight and OH(g) are not present indoors,
but HO 2 (g) sometimes is, and it is the most likely
indoor chemical breakdown source of formaldehyde.
Formaldehyde is also removed by deposition to the
ground and reaction with wall, floor, and ceiling sur-
faces. Health effects of formaldehyde are discussed in
Section 4.2.6.

9.1.7. Radon
Radon (Rn) is a radioactive but chemically unreac-
tive, colorless, tasteless, and odorless gas that forms
naturally in soils. Its decay products are believed to
be carcinogenic and have been measured in high con-
centrations near uranium mines and in houses, particu-