Radon

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

Sources

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

Effective radon (Rn - 222) Content of Soils SoilsRange of Emanation Coefficient Crushed rocks0.005 – 0.40 Soil0.03 – 0.55 Soil0.22 – % to 20 % of dry weight Sand0.06 – 0.18 Sandy loam0.10 – 0.36 Silty loam0.18 – 0.40 Heavy loam0.17 – 0.23 Clay0.18 – 0.40 Soil0.09 – 0.10Dried at 105°C for 24 h Uranium ore0.06 – 26Saturated with water Crushed Uranium ore0.055 – 0.55Saturated with water Tailings from Uranium plant0.067 – 0.072Dried at 110° C Source: Nazaroff et al., 1988

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

Permeability of Soils Soil typePermeability (m² / h) Clay1 x 10 E (- 16) Sandy clay5 x 10 E (- 15) Silt5 x 10 E (- 14) Sandy silt and gravel5 x 10 E (- 13) Fine sand5 x 10 E (- 12) Medium sand1 x 10 E (- 10) Coarse sand5 x 10 E (-10) Gravel1 x 10 E (- 8) Source: Terzaghi, 1967: Tuma, 1973

Sources of Radon  Water is also one of the potential sources due to high solubility of radon  The transfer of radon from water to air decides its contribution to the indoor concentration  Building materials like granite, clay bricks, marble and sandstone are also sources of radon  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  Radon is measured indoors by the detection of alpha, beta or gamma emissions during the decay  The sampling methods are classified as:  Grab sampling 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  Continuous sampling  This method gives a real time measurement at short interval over a long time  The devices available for this type are:  Flow through scintillating chamber (two-port Lucas cell)  Solid state detector (wrenn chambers)  The wrenn chamber is the most widely used device capable of measuring concentrations even below 10Bq/m³

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

Radon concentration calculation  Radon concentration is calculated by:  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 4pCi/L as directed by EPA  Charcoal canister is ineffective for radon below 10Bq/m³

Efficiency of Radon Detection recommended by the EPA

Control Strategies

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

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

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

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

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

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

Electronic air cleaners and Increased ventilation  These cleaners have the capacity of reducing the radon gas and the potential alpha energy concentration (PAEC) by a factor of 2 – 20  After various studies combination of ion generator with ceiling fan produced best results (87% reduction)  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  Simple, but rather effective technique is to increase the ventilation rate  For homes with large crawl spaces mechanical ventilation is adopted to decreasge the radon entry into the building (four fold decrease)

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