RadonRadon

IntroductionIntroduction

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

SourcesSources

Sources of RadonSources 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 SoilsEffective radon (Rn - 222) Content of Soils

Soils Range of Emanation Coefficient

Crushed rocks 0.005 – 0.40

Soil 0.03 – 0.55

Soil 0.22 – 0.32 13 % to 20 % of dry weight

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

Factors affecting transport of Radon to the Factors affecting transport of Radon to the surfacesurface

Soil permeability

Porosity

Water content

Temperature

Pressure difference between soil and building structure

Permeability of SoilsPermeability 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 RadonSources 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 MeasurementSampling and Measurement

Sampling MethodsSampling 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 MethodsSampling 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 SamplingIntegrated 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 calculationRadon 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 Efficiency of Radon Detection recommended by the EPAthe EPA

Method Number of Average Error Range of Company ErrorTests (percent) (percent)

Alpha track detector 10 25 11 to 55Activated-charcoal adsorption detector 256 19 1 to 133Continuous radon monitor 99 25 0 to 658Continuous working level monitor 75 40 0 to 1353Electric ion chamber 127 31 5 to 486Grab sampling radon 66 18 3 to 75Grab sampling working level 58 29 3 to 328Radon progeny integrated sampling unit 4 27 1 to 80Source: GAO, 1989

Control StrategiesControl Strategies

Source removalSource 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 considerationsNew 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 pathsSource 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 Sealing agents available and their characteristics characteristics

Caulking agents

Paints

Membranes

Cement-type materials

The sealants used should be moisture resistant

Paints for walls.

Sunslab ventilation 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 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 ventilationElectronic 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 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)