ANALYSIS OF INDOOR RADON CONCENTRATIONS IN HILLSBOROUGH
COUNTY fFLORIDA) PUBLIC SCHOOL BUILDINGS
Kaiss K. Al-Ahmady, Donald M. Phillips, Clark P. Eldredge
State of Florida, Department of Health and Rehabilitative Services
officeof ~nvironmentalToxicology, Radon and Indoor Air Toxics
Tallahassee, FL

ABSTRACT

Indoor radon concentrations have been measured and reported in Hillsborough County, Florida, public
school buildings in response to the Florida mandatory radon testing program for public facilities. Testing of indoor
radon was conducted in 1079 school buildings in 160 public schools belonging to the Hillsborough County School
District. Testing was completed between 1989 and 1994 in compliance with the first five-year cycle of the Florida
mandatory indoor radon testing program, covering 100% of the occupied spaces. Data were collected and organized
into databases by the Florida Department of Health and Rehabilitative Services, the agency responsible for
administering radon regulations in Florida. Analysis of reported results were performed to evaluate indoor radon
concentrations in the county public schools with respect to other indoor radon concentrations in non-residential
buildings in the county and statewide, and with non-residential buildings that are located in the same zip codes as the
schools. The overall average indoor radon concentrations in the schools is 0.7 pCi/I, with 1.6% of the buildings
exhibiting concentrations exceeding 4 pCi/1. The school's overall average is approximately 41.7%, 12.5%, and 0%
less than the average indoor radon concentrations in non-residential buildings located in the same zip codes of the
county, countywide, and statewide; respectively. The majority of school buildings exhibited maximum to average
indoor radon concentration ratios equal to or less than two.

INTRODUCTION
Beginning in 1989, all of the following buildings have been required to be tested for indoor radon: all
public and private school buildings or school sites housing students in Kindergarten through grade 12; all stateowned, state-operated, state-regulated, or state-licensed 24-hour care facilities such as hospitals, detention centers,
and nursing homes; and all state-licensed day care centers for children or minors. The mandatory indoor radon
testing program was established by a 1988 Florida Legislature action, and administered by the Florida Department of
Health and Rehabilitative Services (HRS) through Chapter 10D-91 of the Florida Administrative Code (FAC),
(FHRS 1994).
Chapter 10D-91 established periodic testing cycles of 5-year each. During the first testing, a facility is
required to be test for loo%, 20%, and 10% of the occupied spaces on the first, second, and third floors;
respectively. These spaces must be habitable and occupied on a regular basis. Occupied space testing coverage is
then reduced to 20% for the next cycles if no location in the building exhibited indoor radon concentrations greater
than 4 pCi/1. Although mandatory testing for indoor radon concentrations in residential buildings have been
conducted following the United States Environmental Protection Agency (USEPA) short-term testing protocols,
testing in non-residential buildings have been conducted following the FAC measurement procedures. The latter
procedures were developed to provide uniform protocols for the purpose of the mandatory testing in Florida.
Measurement procedures are similar to the USEPA short-term testing protocols, however, it addresses several issues
that have a significant impact on indoor radon such as the heating, ventilating, and air-conditioning systems (FHRS
1994). In this work, results of the first-cycle indoor radon testing (1989-1994) for the Hillsborough County public
school buildings, reported to the Florida HRS through the mandatory testing program, are presented and analyzed.

1996 International Radon Symposium 1- 6.1

APPROACH

Testing of Hillsborough County School District (HCSD) public school buildings was performed by GLE
Associates, Inc., for the School Board using activated charcoal adsorption devices. The devices were deployed for a
minimum period of 48 hours, approximately 3 to 5 feet above the floor and a minimum of 1.5 feet from any wall, and
as much near the center of the room as possible. One test device was placed in each room, and where rooms are
greater than 2000 square feet, additional devices were placed. Duplicates and blanks were used for the quality
control requirements of the measurement procedures. One blank devise, not exposed at the site, is randomly selected
from every 20 devices and analyzed to indicate background in the devices and to monitor for other potential
problems that might be generated as a result of shipping, storage, or processing of the measurement devices. A
duplicate device is used with every 10 devices used at a site for the purpose of comparing the measurement results.
Plumbing, electrical conduit and ventilation penetrations through the floor into each room and ventilating
fans and chases carrying plumbing or other conduits, which are visible, were noted and identified prior to the testing.
Building floor plans were used to identify test device locations according to the testing protocols outlined in Chapter
10D-91 of the FAC. Tests were conducted in each building by trained technicians. Results of HCSD school
buildings reported between 1989 and 1994 were organized into databases by the Florida HRS. Related indoor radon
concentrations for other non-residential buildings reported to HRS under the mandatory testing program, using the
same measurement procedures, and mostly conducted by trained technicians, were organized into databases based on
the zip codes in the county in which the county's public schools are located for the purpose of comparison. Other
non-residential indoor radon testing data for Hillsborough county and for the state of Florida, were also organized
and used in the analysis.
RESULTS AND DISCUSSION

There are 160 schools in HCSD that were tested for indoor radon concentrations during the f i r t cycle of the
Florida radon mandatory testing program. These schools compromise 1079 non-residential buildings, bringing the
average number of buildings per school to approximately 6.7. Most of these buildings are one story structures,
however; their shape, complexity, size, and slab and foundation details varied greatly. The number of devices used
to measure indoor radon concentrations in each building ranged from one to 269 devices, at an average of 12.3
testing devices per building.
Since data used in this analysis are collected only from the first testing cycle results, testing was performed
to cover 100% of the occupied spaces. The schools are located in geographical areas spanning 44 zip codes in the
county. Indoor radon measurements performed in other non-residential buildings, under the mandatory testing
program, were collected and organized for buildings located in the same zip codes set where the schools are located.
Table 1 summarizes results obtained from the schools along with results obtained from other buildings in the zip
codes set, the Hillsborough County, and the State of Florida.
As seen in Table 1, buildings belonging to the HCSD have an overall average indoor radon concentration of
approximately 0.7 pCi/1 with only 1.6% of the buildings exceeding 4 pCV1. This average is approximately 41.7%
less than the overall average indoor radon concentration collectively obtained from reported non-residential
buildings in the same zip code areas where HCSD buildings are located. This significant difference is attributed to
an extra measure undertaken by the Hillsborough County School Board to reduce indoor radon concentrations in
their school buildings by employing different mitigation techniques. The latter were gradually installed in some
school buildings during the past five years. It is expected that calculations of the overall average indoor radon
concentration exhibited in HCSD buildings based on the second five-year cycle reporting requirements will result in
further reductions, compared to the non-residential buildings in the area, as more radon mitigation systems are
further utilized and installed.

1996 International Radon Symposium I - 6.2

Table 1: Summary of indoor radon measurement results and the percentage of buildings exhibiting concentrations
greater than 4 pCiA for HCSD school buildings with respect to other relevant data.
Average Indoor
Rn
( P C ~

Item

1

Hillsborough County School District

Maximum
Indoor Rn
(PCW

Buildings
Exceeding
4 pCi/l

(%I
1.6

--

Non-Residential Buildings in HCSD zip code areas

1 Non-Residential Buildings in Hillsborough County
1 Non-Residential Buildings in Florida

4.8
5.1
6

Using indoor radon concentration data reported under the Florida mandatory testing program in
Hillsborough County, and in the State of Florida, Al-Ahmady et. al. calculated the overall average indoor radon
concentrations as 0.8 and 0.7 pCV1 for countywide and statewide, respectively (Al-Ahrnady et. al. 1996). These
figures are in very good agreement with the current estimate of the indoor radon concentrations in the HCSD
buildings. The latter is still 12.5% smaller than the estimate of the average indoor radon concentration countywide.
While the average concentration in the HCSD equals the statewide average indoor radon concentration in nonresidential buildings (0.7 pCVI), the percentage of school buildings exhibiting concentrations greater than 4 pCiA is
significantly less at 1.6% compared to the statewide estimate of 6%. Further, the number of HCSD buildings
exceeding 4 pCV1 are only 33% and 31% of the non-residential buildings with indoor radon concentrations
exceeding 4 pCV1 in the same set of zip code area where schools are located and in the entire county, respectively.
This is also attributed to the efforts undertaken by the Hillsborough County School Board to control radon exposure
in their school buildings.
Figure 1 shows the distribution of reported indoor radon concentrations in HCSD school buildings and in
non-residential buildings located in the same zip codes where schools are located, plotted as a percentage relative to
the range of average indoor radon concentrations. As seen in the figure, over 60% of school buildings exhibiting
concentrations less than one pCi/l. The number of school buildings sharply drops as the range of average
concentration increases to about 3.5 pCfl, and almost became negligible when the range increases to over 4 pCi/l.
The corresponding percentage of non-residential buildings located in the same zip codes, where schools are located,
is less by more than 10% for the range of average indoor radon concentrations less than one pCV1. However, the
number of buildings located in the same zip codes as the schools and exhibiting concentrations in ranges greater than
1.5 pCV1 are systematically larger. Yet, the general trend of these buildings show a sharp decrease as the average
indoor radon concentrationsexceed 0.5 pCV1, similar to the trend of the school buildings.
Figure 2 shows the distribution of the percentage of buildings versus ranges of reported maximum indoor
radon concentrations. These concentrations are comparable to each other with levels of 31.6, 32.9, and 34 pCiA for
the HCSD school buildings, non-residential buildings located in the same zip codes, and non-residential buildings
countywide; respectively. However, they are at least 60% less than the maximum indoor radon concentration in nonresidential buildings statewide, reported at 86.1 pCfl (Al-Ahamdy et. al. 1996). Over 80% of HCSD schools have
maximum indoor radon concentrations less than or equal to 2.5 pCi/I. This is significantly larger than the percentage
of non-residential buildings countywide, which is less than 30%. The percentage of school buildings decrease as the
range of maximum indoor radon concentrations increase, and exhibited sharp drop when ranges exceed 2.5 pCi/l.
This wend is similar to the trend observed with ranges of average indoor radon concentration. However, unlike the
school buildings trend with average indoor radon, their trend with maximum indoor radon noticeably differs from the
wend of non-residential buildings countywide. Although, a decrease in the latter can be easily observed when ranges
1996 International Radon Symposium I - 6.3

of maximum indoor radon concentration exceed 5 pCV1, neither the peak percentage value nor the trend at higher
concentrations is similar to the HCSD school buildings.
Figure 3 illustrates the relationship between the range of maximum to average indoor radon concentrations
in HCSD buildings and the count of the school buildings. The highest number of buildings is shown to have
maximum indoor radon concentrations that are twice the average concentrations exhibited in these buildings.
Further, the number of school buildings that exhibited maximum to average indoor radon concentrations less than or
equal to 2, exceeds the rest of buildings that exhibited ratios greater than 2. It is also interesting to notice that the
number of school buildings exhibiting maximum to average indoor radon concentration ratios that are whole
numbers, systematically exceeds the number of buildings in that range of maximum to average concentration. This
phenomenon is likely to be an artificial effect that is generated from the tendency to round up indoor radon
measurement results during processing and reporting.
CONCLUSIONS

Results of indoor radon testing performed at 1079 public school buildings belonging to 160 public schools
in Hillsborough County, Florida, reported in this study indicate that these schools exhibiting average indoor radon
concentrations significantly less than concentrations obtained from non-residential buildings located in the same zip
code area where the school are located. Furthermore, the number of HCSD school buildings that reported with
average indoor radon concentrations exceeding 4 pCV1 is significantly less than the number of non-residential
buildings located in the same zip codes as the schools, across the county, and statewide. This is attributed to extra
measures undertaken by the county school board to reduce indoor radon concentrations in their school buildings by
installing radon mitigation systems. The maximum indoor radon concentrations in the schools were observed to be
similar to concentrations exhibited in non-residential buildings located in the same zip code areas, where the schools
area located, and in the entire county. However, all maximum concentrations reported were at least 60% less than the
value of the statewide non-residential maximum indoor radon concentration. It has been observed that similar trends
exist as the percentage of reported buildings sharply decrease as the average indoor radon increases to a specific
level for both school buildings and other non-residential buildings in the county. Although, a very sharp decrease in
the percentage of HCSD buildings occurs when maximum indoor radon concentrations, measured in these buildings,
exceed a specific level; a different trend is observed for other non-residential buildings in the county. It has been
shown that a possible artificial effect may occur when indoor radon measurement data are reported or processed.
The latter phenomenon was remarkably noticed as the maximum number of buildings systematically occurred at
maximum to average indoor radon concentration ratios forming whole numbers for each range of the ratios.
REFERENCES
Al-Ahrnady, K.K.; Klein, W.G.; Eldredge, C.P.; Phillips, D.M.; Gilley, N.M."Study of the Reported Indoor Radon
Measurement Results for Residential Structures and Large Buildings in Florida", In Proceeding of the 1996
International Radon Symposium, Haines City, Florida, September 29- October 2; 1996.
Florida Department of Health and Rehabilitative Services (HRS) "Control of Radiation Hazard Regulations- Radon",
Florida Administrative Code, Chapter 1OD-9 1; 1994

-

1996 International Radon Symposium I 6.4

1

1

HC School Dbtrtct

3

2

1

0

4

5

7

6

Range of Avoroge Indoor Radon (pCI/l)

.

10

9

8

Figure 1: The percentage of HCSD school buildings and non-residential buildings in Hillsborough County as a
function to their average indoor radon concentrations.

90 School District

/HC

80

70

I

85

-1:
0

0

5

10

15
Ran-

20

25

30

35

---

40

of Max. Indoor Radon (pCI/I)

Figure 2: The percentage of HCSD school buildings and non-residential buildings in Hillsborough County as a
function of their maximum indoor radon concentrations.

-

1996 International Radon Symposium I 6.5

Figure 3: The frequency of occurrence of HCSD school buildings as a function to their reported maximum to average
indoor radon concentrations.

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1996 International Radon Symposium I 6.6