Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

Airborne Radioactivity
RADIATION SAFETY GUIDES 357
For purposes of computing concentration standards for airborne radioactivity, atmospheric
contaminants may be broadly classified as gaseous and particulate. Radiation
exposure from airborne radioactivity may occur via three pathways:
External exposure (submersion dose) from radionuclides outside the body.
Internal exposure due to radionuclides inhaled into the body.
Internal exposure due to ingestion of radionuclides that had entered into the food
chain as a result of fallout from the air.
The submersion dose is the limiting factor for acceptable atmospheric concentrations
only for several biochemically inert gases, such as argon, krypton, and xenon.
For all other airborne radionuclides, the internal dose is the limiting factor. For inhaled
radioactivity, the lung is considered from two points of view: as a portal of entry
for inhaled substances that are absorbed into the body from the lung and are then
systematically transferred to one or more organs, and as a critical organ that may
suffer radiation damage. Inhalation is a major portal of entry for noxious substances
because the amount of material taken into the body by inhalation is far greater than
by ingestion, as shown in Table 8-5.
The hazard from a toxicant depends mainly on two factors: its inherent toxicity
and on the probability of a toxic amount reaching the site of its toxic action.
The hazard from inhaled radioactive dusts—or indeed the hazard from any toxic
material—must include a consideration of the likelihood that the toxic substance
will reach the site of its toxic action. In the case of inhaled radioactive dusts, this
site is assumed to be the bronchial epithelium, the alveolar epithelium, and the
pulmonary lymph nodes. The two main factors that influence the degree of hazard
from toxic airborne dusts are (1) the site in the lung where the dust particles are
deposited and (2) the retention of the particles within the lung.
Particle-Size Distribution
Dusts generated by almost any process are found to be randomly distributed in size, or
“diameter,” around a mean value. This size distribution is found to be “log-normal.”
The logarithm of the particle size is found to be normally distributed, rather than the
size itself. In this case, the mean size, which is called the geometric mean, is defined by
n log dn
mg = antilog
, (8.11)
n
where dn is the particle diameter (which most often is expressed in microns,
1 μm = 10 −4 cm) and n is the number of particles that are being sized. The
TABLE 8-5. Amounts of Material Taken into the Body via Inhalation and Ingestion
MEDIUM VOLUME (L) WEIGHT (kg)
Food Not applicable 1.9
Water 2.2 2.2
Air 20,000 26