EXPOSURE ROUTE
- inhalation- virtually all human exposure occurs through the respiratory system
- ingestion - minimal exposure
- dermal - no significant exposure
- delivered human dose from radon exposure cannot be measured directly
- radon exposure assessment is modeled beginning with measured air concentration
RADON CONCENTRATION IN AIR
1) Radon-222 gas:
- is measured by its activity or rate of decay in units called "curies"
- One curie equals 3.7 x 10(7) radioactive disintegrations per second
- One pico curie (pCi) equals 3.7 x 10(-2) radioactive disintegrations per sec
2) Solid radon-222 decay products (primarily polonium 218 and 214)
- ratio of solid radon-222 progeny to radon-222 gas in air is 0.2 - 0.8
- solid radon-222 progeny deliver the actual radiation dose to lung tissue
- radiation dose of radon-222 decay products is expressed as "working levels" (WL's)
- one WL equals a combination of radon decay products which releases 1.3 x 10(5) MeV of potential alpha energy (equivalent to the decay of 100 pCi/L of radon-222)
- cumulative exposure is expressed in WLM_s (working level
months) for exposure to one WL for one working month (170 hours)
RECOMMENDED EXPOSURE LIMITS FOR RADON-222
1) Miners Safety and Health Act (underground miners)
- No worker exposure to air containing 1.0 WL (100 pCi/L) of radon progeny
- Annual exposure limited to no more than 4.0 WLM per year
2) Occupational Safety and Health Administration (all other covered workers)
- exposure limited to 30 pCi/L or or 0.33 WL based on
continuous workplace exposure of 40 hours per week and 52 weeks per year
3) Environmental Protection Agency (residential recommendations)
- year-long average radon exposure of less than 4.0 pCi/L
(0.04 WL) in any livable area of a home
POTENTIAL DOSE OF RADON-222 DECAY PRODUCTS
- potential dose = radon concentration in air times intake rate of media (air) and duration of exposure
- potential dose is affected by respiratory rate, lung tidal volume and bronchial morphology as well as aerosol characteristics
- a high percent of airborne radon decay products attach to ambient aerosols
- quantity, size and density of airborne particles affect deposition in respiratory tract
- intake rate of media (air) is approximately 0.5 L/Min or 20 m(3)/day
INTERNAL (ABSORBED) DOSE OF RADON-222 DECAY PRODUCTS
- absorbed dose = potential dose x absorbtion factor
- absorbtion factor is affected by mucous thickness and clearance rate
- radiation absorbed dose is defined as energy deposited per unit mass of tissue
- radiation absorbed dose is expressed in "rads" or "greys" (Gy)
- 1 rad = 100 ergs/gram, 1 Gy = 100 rads
- absorbed dose is modified by a qualitative factor related to linear energy transfer
- modified absorbed dose is measured in "rems" (roentgen equivalent man)
- the average effective dose equivalent exposure of ionizing radiation from all sources to the US population is estimated at 300 mrem / year
- radon decay products account for 200 mrem / year of this total (NCRP Report #94)
