Radon Introduction Radon is a colorless and odorless

Radon

Introduction Ø Radon is a colorless and odorless gas produced by the decay of radium – 226 Ø Radon after decay produces radioisotopes known as radon daughters Ø Radon progenies (Po-218 and Po-214) are of health concern, as they tend to retain in the lungs causing cancer Ø The upper limit recommended by US EPA for radon is 4 p. Ci/L Ø Radon is found in many states in the USA

Sources

Sources of Radon Ø Sources of radon include v Soil v Rocks beneath or surrounding the building v Water v Building materials v Natural gas Ø Radon from soil moves slowly from the pores of the soil to the surface by diffusion or pressure induced flow Ø Radon enters the building from the cracks and joints in the foundation

Effective radon (Rn - 222) Content of Soils Range of Emanation Coefficient Crushed rocks 0. 005 – 0. 40 Soil 0. 03 – 0. 55 Soil 0. 22 – 0. 32 Sand 0. 06 – 0. 18 Sandy loam 0. 10 – 0. 36 Silty loam 0. 18 – 0. 40 Heavy loam 0. 17 – 0. 23 Clay 0. 18 – 0. 40 Soil 0. 09 – 0. 10 Dried at 105°C for 24 h Uranium ore 0. 06 – 26 Saturated with water Crushed Uranium ore 0. 055 – 0. 55 Saturated with water Tailings from Uranium plant 0. 067 – 0. 072 Dried at 110° C Source: Nazaroff et al. , 1988 13 % to 20 % of dry weight

Factors affecting transport of Radon to the surface Ø Soil permeability Ø Porosity Ø Water content Ø Temperature Ø Pressure difference between soil and building structure

Permeability of Soils Soil type Permeability (m² / h) Clay 1 x 10 E (- 16) Sandy clay 5 x 10 E (- 15) Silt 5 x 10 E (- 14) Sandy silt and gravel 5 x 10 E (- 13) Fine sand 5 x 10 E (- 12) Medium sand 1 x 10 E (- 10) Coarse sand 5 x 10 E (-10) Gravel 1 x 10 E (- 8) Source: Terzaghi, 1967: Tuma, 1973

Sources of Radon Ø Water is also one of the potential sources due to high solubility of radon Ø The transfer of radon from water to air decides its contribution to the indoor concentration Ø Building materials like granite, clay bricks, marble and sandstone are also sources of radon Ø Fly ash from coal-fired power plant is a major source of radon, which is used in concrete and cement

Sampling and Measurement

Sampling Methods Ø Radon is measured indoors by the detection of alpha, beta or gamma emissions during the decay Ø The sampling methods are classified as: v Grab sampling ü ü ü The study is conducted for a short period indoors by using scintillating flask This method is advantageous in sensitivity and rapidity but is less accurate When concentration is less than 10 Bq / m³ the error is more than 30%

Sampling Methods Ø Continuous sampling v This method gives a real time measurement at short interval over a long time Ø The devices available for this type are: v Flow through scintillating chamber (two-port Lucas cell) v Solid state detector (wrenn chambers) Ø The wrenn chamber is the most widely used device capable of measuring concentrations even below 10 Bq/m³

Integrated Sampling Ø The devices used in this technique are: v Alpha tract detectors v Electronic ion detectors v Charcoal canisters Ø The charcoal canister method is EPA recommended and widely used method Ø This is easy to use and can be sent through mail to lab for analysis Ø The disadvantage of this method is an assumption that charcoal never reaches an equilibrium with the atmospheric radon

Radon concentration calculation Ø Radon concentration is calculated by: Ø Rn = {net CPM} / { T(s) (E) (CF) (DF)} Where CPM – counts per minute T(s) – exposure time E – efficiency of detector CF – calibration factor DF – decay factor This method is effective for measuring concentrations above 4 p. Ci/L as directed by EPA Ø Charcoal canister is ineffective for radon below 10 Bq/m³

Efficiency of Radon Detection recommended by the EPA

Control Strategies

Source removal Ø Selection of construction sites having low radium content Ø Knowledge of local soil characteristics such as permeability and moisture content Ø Removal and replacement of soil from a perimeter of 3 m from the building foundation Ø The cost for this process is site specific and can range from $5, 000 to $20, 000

New construction considerations Ø Radon concentration can be substantially reduced by new construction techniques Ø Provision of soil gas outlet to the sun slab and crawl spaces Ø Increasing the permeability by placing minimum of 4 inches of aggregate under slab Ø Double barrier approach can be used for slab-on-grade and crawl space construction

Source Control by sealing Entry paths Ø Floor drains and sumps connected to drainage systems Ø Openings around utility lines Ø Hollow concrete block walls Ø Junction between walls and floor and slab Ø Cracks in building materials Ø Exposed soil and rocks having radon Ø Unpaved crawl space

Sealing agents available and their characteristics Ø Caulking agents Ø Paints Ø Membranes Ø Cement-type materials Ø The sealants used should be moisture resistant Ø Paints for walls.

Sunslab ventilation Ø The design of sunslab ventilation is house specific and depends on nature of foundation Ø Fan with a capability to create 50 – 100 Pa is installed on end of the pipe running from the basement Ø This can be made effective by placing multiple collection ports for each wall Ø This is good for old structures, but excessive cracks diminish its effectiveness Ø This is very effective if drain tiles surround the entire house

Basement pressurization and Air cleaning Ø This method is highly effective method if the basement is airtight Ø Over pressurization of the basement drastically reduces the radon concentration below 4 p. Ci / L Ø This method is disadvantageous where there is increased ventilation and excessive windows and doors activity Ø This is one of the ways of reducing the radon concentration Ø During this process the air exchange rates are increased using the HVAC systems Ø Increased ventilation and activated carbon beds can remove the radon gas and its daughter products

Electronic air cleaners and Increased ventilation Ø These cleaners have the capacity of reducing the radon gas and the potential alpha energy concentration (PAEC) by a factor of 2 – 20 Ø After various studies combination of ion generator with ceiling fan produced best results (87% reduction) Ø Another way of decreasing the radon from indoors is plate-out i. e. by pushing the charged progenies to walls or floors and then outdoors Ø Simple, but rather effective technique is to increase the ventilation rate Ø For homes with large crawl spaces mechanical ventilation is adopted to decreasge the radon entry into the building (four fold decrease)

Adsorption Ø The radon adsorption can be another way in reducing its concentration and depends on following factors: v Air flow rates v Radon concentration v Relative humidity Ø Activated carbon is used as adsorbent (having high capacity for radon and minimum interference with moisture and other VOC’s)