REGIONAL INFLUENCES OF CONSTRUCTION ON THE EFFECTIVENESS OF RADON CONTROL

INTRODUCTION

Within the last three years indoor radon has been identified as a serious health threat to a substantial portion of the United States population. This danger has forced a new awareness on the construction industry of the degree of safety provided to the American consumer by new and existing construction. Major efforts are currently underway by national and state agencies to better understand the nature of the radon problem. It appears that radon is camron to most parts of the US but is found in higher concentrations in particular geologic conditions. Indoor radon concentrations are a function of the geologic source condition, the number and size of penetrations through the conditioned envelope and the magnitude of the driving force, natural or mechanical. .r11 Regional mitigative solutions are being developed and tested at the University of Florida, as well as, techniques to be utilized in new construction for the control of radon gas. This paper will discuss this research with a particular focus on the aspects of construction education which will influence the ability of the construction graduate in dealing with this and future problems of similarity.

RADON

Radon is a odorless, colorless and tasteless radioactive gas. It is produced from the radioactive decay of uranium and radium, elements found worldwide in almost all soil formations. r 11 Radon is one of the Noble Gasses which characterizes it as being neither atomically attractive nor chemically reactive with other substances. Once radon is produced in the soil it must migrate from its place of origin to an indoor environment within its 3.8 day half life in order for it to become a human health hazard. Once inside a closed environment radon may be inhaled and exhaled by occupants of that space with little health risk. If, however, the radon atom decays while within the respiratory system, then the greatest health risk would occur. After radon decays, its "progeny" continue the decay process until, within a very short time frame, Lead 210 is formed. Lead 210 is considered the end of the radon hazard chain. These "progeny" or "daughters" include Polonium 218 (3 minute half-life), Lead 214 (27 minute half-life), Bismuth 214 (19.7 minute half-life) and Polonium 214 (0.00016 second half­life). These four isotopes, created by the successive decay of Radon 222, take less than a total of 50 minutes to transition from Radon 222 to Lead 210. (See Figure 1) Each of these isotopes emit radioactive energies when they decay. Sane of these isotopes, including Radon 222, give off alpha energy, or particles, which is the most hazardous of these energy types to sensitive lung tissues. Other important characteristics of these "progeny" are that they are particles, not gasses, and are electrostatically charged. These charged particles tend to stick or "plate out" to other particles or surfaces when they come into contact with them. When radon decays into Polonium 218 and this particle comes into contact with smoke or dust particles in the air the electrostatic charge causes the radioactive polonium isotope to adhere to the other particle. If subsequently inhaled and not exhaled for 50 minutes or more the surrounding lung tissues will be repeatedly bombarded by the radioactive energies associated with the decay process of these isotopes. This repeated exposure of unprotected tissues to alpha energies is what generates the specific health hazard associated with radon gas.

REGIONAL ASPECTS OF RESIDENTIAL CONSTRUCTION

Residential construction differs substantially from commercial and institutional construction due primarily to influences brought by the constructor in response to local or regional forces. Sane of these forces include the style of a particular area, or the use of native materials in the construction, or of the particular difficulties of dealing with local geologic or climatic conditions. Where there are many similarities in how we construct residential structures there are several unique differences which can be considered to be of regional influence. For the remainder of this paper the authors will discuss the effects of the regional construction variations of the Sunbelt Region (Florida) and the Northern Region (Pennsylvania, New York, etc.).

The first regional difference effecting radon and construction is the local geology. Where much of the northern construction is built aver granite bedrock the sunbelt area is predominately comprised of sands and clays overlaying limestone. Granite is known, in same areas, to have heavy uranium mineralization. As long as uranium is embedded in the granite matrix the radon produced by its decay will be trapped. When granite fractures the radon that is produced along the surface of the fissure will accumulate and the fracture will function as a duct to channel the radon. High concentrations are usually found in these crevices and when a structure is constructed on top of it the likelihood of having elevated indoor levels is very significant. The Stanley Watras house, the well known origin of today's radon crusade, was confirmed to have been constructed over a bedrock fissure.

In Florida the soils containing the radon parent sources are predominantly highly mineralized clays.

No radon problem has been identified solely with the limestone base formation. These clays are associated with several of Florida's geologic soil formations, particularly the Hawthorn, Bone Valley and Alachua formations and the hardrock phosphate present along the Ocala Upliftr (See Figure 2).

Most of Florida's soil formations are prised of horizontal sedimentary strata of clays and sands.

The mineralized clays containing uranium and radium, for the most part, are covered by additional layers of non mineralized clays and/or sands. This natural condition is prevalent throughout most of the radon prone area in Florida but in the phosphate mining areas of the state the radon problem is a function of these soils having been disturbed by the mining process and left in a blended state. It is currently thought that if the radon producing soils are at a depth greater than five feet below the surface then radon migration will be sufficiently delayed to minimize the risk of elevated indoor levels. This assumption does not apply to those sites where the natural soils have been disturbed, as in Polk County, Florida where development on reclaimed phosphate mining sites is common. In natural conditions where enriched clay strata surface along erosion sites, such as stream channels, and at the geologic boundary the radon potential becomes very significant. Hones located on these soils have been measured to have indoor levels exceeding 150 pCi/l. The eruption of these clay layers to the surface can frequently be mapped with a reasonable degree of accuracy. This mapping has been completed in Alachua County, Florida, and illustrates that lands between 105' MSL (mean sea level) and 145' MSL are at a much higher risk of having a radon problem. Where the exact boundary of this mineralized clay layer varies from one site to another it has made the builders task of knowing when to respond to a potential radon problem much easier.

Probably the most influential regional characteristic on construction is the local climatic conditions. Temperature variations and duration, precipitation rates, locally hazardous weather conditions are but a few of the climate factors influencing construction. Frost lines vary as a function of climate and result in the regional differences of foundation construction. Basements of the northern region are a response to these adverse climatic conditions and result in substantially different radon entry conditions and control approaches. Where structures are built into the ground, basements, the exposure to radon is greatly enhanced. In the northern region it is common to construct housing with a subgrade space extending at least six feet below grade. Typical Florida construction practice is to construct a slab-on-grade structure thereby taking advantage of any soils overlying the radon producing formation. Subgrade basement construction not only increases the proximity to the radon source but increases the exposed building area by a large degree.

Climate also influences the style of construction as well as the degree of concern for weatherization. Hones built in more northern climatic zones have traditionally been constructed in a vertical orientation for more effective use of heated air and are typically sealed tighter than Florida housing. The basement penetration into the gram d not only increases the entrance opportunity for radon but has, in multi-story structures, has created a "stack effect" which has been shown to effectively depressurize the basement area. r31 When hot air rises the resulting low pressure area created by the vertical movement of the hot air results in a measurable driving force for soil gasses such as radon. It is suspected that the increased soil contact coupled with the increased stack effect, of northern style hones, is the reason that large variations in indoor radon concentrations between northern and southern hones has been recognized when the subslab radon levels are similar. In Florida's style of construction, it is rare that sub-grade spaces are created and normally single-story ranch style homes are constructed which do not produce a significant stack effect condition.

The predominate foundation/floor system used in the Sunbelt Region is a slab-on-grade system. An important regional difference in the way slabs are constructed is that basement slabs must be constructed to accommodate water removal. Stone is normally placed under basement slabs in order to facilitate horizontal water movement to a sump. This stone provides a very permeable substrate for the slab and can also be used as a ventilatable substrate for radon control. The sub-slab depressurization method of mitigation, which intercepts radon before it enters the occupied space, has been chosen by USEPA as the mitigation system of choice for slab construction. u 'Luis technique was expected to perform satisfactorily with Florida's slab construction but has met with less than satisfactory results. The use of sand fill under slabs instead of stone fill is the major contributing factor in the reduced effectiveness of this technique. Sand aggregate has significantly less permeability than rock substrate and is thought to be severely influenced by ground moisture. In fine grain soils the wicking of ground water replaces the air in the void space between the sand grains and reduces the ventilatable area. Groundwater is primarily a function of rainfall. Florida is subjected to a frequency of rainfall that makes the use of sand fill, as a ventilatable media, questionable. Early demonstrations projects in Florida by USEPA have shown that the sub-slab depressurization technique can work to lower indoor radon but, it has also shown that the system effectiveness is significantly reduced by the influence of rainfall.

Mechanical system location is another construction difference associated with region. Where basements are used it is cannon practice to locate the air handler, for forced air systems, in this space. Most air handling equipment is poorly sealed, and when the system is in operation it draws roan air into the unit and then distributes it throughout the house. When elevated levels of radon are present in the basement the mechanical system circulates this contaminated air through the structure. It has been determined that most ducting is not well sealed nor maintained. When return air ducts are located in a contaminated space and leak the result is radon is inducted into the mechanical system and distributed through the house. In Florida construction the air handler is normally located in the garage, in an interior closet or in the attic. All of these locations are less susceptible to elevated radon contamination than in basement construction. However, common techniques of installing the mechanical equipment and duct system, in southern homes, can increase radon entry by inducing negative pressures on the slab. This localized depressurization can provide a significant driving force for soil borne radon where penetrations in the slab are present.

UNIVERSITY OF FLORIDA RESEARCH

For the past three years the University of Florida Indoor Radon Task Force has been actively involved in radon research on both new and existing structures. The Task Force is composed of faculty and graduate students representing five departments from four colleges on campus. Environmental Engineering Science and Nuclear Engineering Science, both from the college of Engineering are the health physics team which supply the measurement and detection capability while the Department of Geology provides soils and geological identification and mapping services. The College of Journalism conducts citizen surveys, prepares press releases and generates other media and informational related materials. The College of Architecture's School of Building Construction provides the construction analysis, planning, design and supervision of mitigation system installation services. Currently, the task force is conducting research to develop, demonstrate and refine various mitigation techniques for the USEPA and the State of Florida Building Construction Industry Advisory Committee (BCIAC). The task force is also engaged in developmental research for the legislatively mandated State of Florida Radon Resistant Building Code for New Construction.

Two of these research projects involve the demonstration of various mitigation techniques on 12-15 houses of differing construction. The BCIAC grant is to diagnose the radon problem on five houses which are not all of typical Florida construction. This grant is allowing us to develop mitigation approaches for those housing types that are either non-slab, combination floor systems types or exceed the average size house typical of Florida. The USEPA grant, supplemented by the state code development work, is studying in detail the conditions and effectiveness of subslab depressurization systems on slab-on-grade houses. There are three categories of slab construction and each one has specific radon entry conditions. These slab types are: the monolithic slab; the floating slab; and the slab cast fully or partially into the stemwall. Figure 3 illustrates the construction differences and the potential entry conditions associated with each of these slab types. Two of the slab types allow for radon to migrate into the superstructure walls. This condition complicates the radon interception concept of subslab depressurization. When radon invades the wall cavity in existing construction, especially when the wall is constructed of concrete block, the only solution is to depressurize the wall cavity. Another regional construction variation is that in Florida contractors have to build to resist hurricane force winds and are required in masonry construction to extend vertical reinforcing steel from the footing to the tie beam. The block cavity where the vertical steel is placed is then poured with concrete which eliminates the possibility of extending a pressure field throughout the entire block wall from one suction point. This them requires multiple suction locations around the perimeter of the structure. Other construction conditions being addressed in this study include subslab ducting. The house being investigated with this problem not only has a subslab return air duct system but also has a floating slab and masonry exterior walls. An effective mitigation approach for this house is still under development but will include termination and relocation of the return air duct and converting the existing duct into a subslab depressurization system. Much of this research is being conducted to better understand the effects, capabilities and limitations of various mitigation systems as influenced by regional construction differences.

Another study is being conducted to determine the capability of sand fill to support sub-slab depressurization. Influences of compaction and fill depth are being investigated in order to effectively plan suction location and construction technique. This work is being conducted for the State of Florida as part of the research required for the development of the new radon resistant building code. Various fill materials and configurations are being studied to determine the effectiveness of each system as well as the cost impact, constructability and inspectability. This project has just commenced and will provide for the construction of four 1200 square foot test slabs. Horizontal pressure profiles will be developed for each suction location and comparisons of pressure versus compaction, a function of fill depth, will be made. The results of this project will significantly enhance our understanding of subslab pressure field extension when completed.

AREAS OF ACADEMIC CONCERN

Most construction education programs are concentrating on educating their graduates with the knowledge of how to effectively manage the construction process. Fundamental and advanced construction techniques are usually taught in order to give the student an understanding of how various materials go together and stay that way. Overall, the construction programs do a very good job at educating the future constructor for surviving in the market place. The identification of a new hazard to the occupants of the structures our profession builds requires that we educators evaluate whether or not we have a role to play. The authors feel that the indoor radon problem has brought into focus a realization that a new interrelated systems approach to the building process must be considered. Not only does the structural system, the roofing system, the foundation system function to hold up the building but they have a interrelationship with the environmental systems and other components to create something larger than the whole. Indoor pressure conditions and their effects on all systems must be understood by the constructor. This education must start within the academic institutions.

CONCLUSION

It seems that every few years a new construction hazard is identified and a public outcry calls for immediate solutions. Radon gas is the problem of the late 1980's. Because litigation is on the increase throughout our society the construction industry has become the prime target in many instances. Environmental attorneys in Florida are demanding the construction industry produce products that protect the health, safety and welfare of the public, even against unknown and unforseen hazards. Radon gas, a naturally occurring phenomenon, has become a precedent setting issue for construction related environmental health litigation. It is no longer acceptable for the construction academic community to concern itself solely with the problems of structure and process. Future construction graduates must be sufficiently knowledgeable of the mechanics of the built environment to lead the construction industry into the 1990's with an ability to respond effectively to new and emerging conditions.

REFERENCES

 

APPENDIX

 

Radon research conducted by the University of Florida Indoor Radon Task Force:

 

Identification of Candidate Houses for the North Florida Indoor Radon Remediation Demonstration, U.S. Environmental Protection Agency, Genevieve S. Roessler, Ph.D., PI, Oct. 1987 - Apr. 1988.

 

Study of Mitigative Techniques in Existing Houses to Control Indoor Radon Exposure in Florida, Building Construction Industry Advisory Committee, Richard A. Forman, PI, Jun. 1988 - May 1989.

 

Florida Radon Mitigation Project, Phase II - North Florida, U.S. Environmental Protection Agency, Charles E. Roessler, Ph.D., PI, Aug. 1988 - May 1990.

 

Field Characterization of Sub-Slab Depressurization Systems on Full Scale Test Slabs Typical of Florida Construction and an Evaluation of Radon Control Systems for Crawlspace Construction, State of Florida: State University System Board of Regents, Richard A. Forman, PI, Jan. 1988 - Dec. 1988.

 

Study of Characteristics Affecting Design and

Performance of Sub-Slab Ventilation Systems in Florida Houses, State of Florida: State University System Board of Regents, Charles E. Roessler, Ph.D., PI, January 1988 - December 1988.

 

Modeling the Sub-Slab Ventilation Systems Applied to Florida Houses, State of Florida: State University System Board of Regents, David Hintenlang, Ph.D., PI, January 1988 - December 1988.