1
The Potential Regulatory
Implications of Strontium
March 2014
Principal Author
Katherine Alfredo, Ph.D., Columbia Water Center
(Former Intern at the American Water Works Association)
Co-Authors
Craig Adams, Ph.D., P.E., F.ASCE, Utah State University
Andy Eaton, Ph.D., BCES, Eurofins Eaton Analytical
J. Alan Roberson, P.E., American Water Works Association
Ben Stanford, Ph.D., Hazen and Sawyer
Strontium - Table of Contents
1. Introduction ................................................................................................................................ 2
1.2 General uses and environmental exposure .......................................................................... 4
2. Occurrence .................................................................................................................................. 5
2.1 Environmental Occurrence.................................................................................................... 5
2.2 NIRS Database ....................................................................................................................... 8
2.3 Ongoing UCMR3 monitoring ............................................................................................... 10
3. Health Effects ............................................................................................................................ 10
4. Technology Assessment ............................................................................................................ 11
4.1 Analytical Methods.............................................................................................................. 11
4.2 Treatment Technologies ..................................................................................................... 11
5. Health Levels of Potential Interest ........................................................................................... 11
6. Concluding Remarks.................................................................................................................. 13
7. References................................................................................................................................ 13
Table of Tables
Table 1: 32 CCL3 contaminants discussed during June 2011 stakeholder meeting (Roberson
2012) ................................................................................................................................... 2
Table 2: Summary of occurrence data for four CCL3 contaminants (adapted from Roberson
2012) ................................................................................................................................... 3
Table 3: Physiochemical properties of strontium (The Merck Index 2006) ................................... 3
Table 4: Chemical identification of strontium and strontium compounds (ATSDR 2004) ............. 4
Table 5: Radioactive strontium isotopes ........................................................................................ 4
Table 6: U.S. distribution of strontium compounds by end of use (by percent) ............................ 5
Table 7: Preliminary strontium UCMR3 monitoring results—11,788 samples from approximately
1,400 utilities (USEPA, 2013) ............................................................................................ 10
Table 8: Sample preparation and analysis methods for strontium determination in water (ATSDR
2004) ................................................................................................................................. 11
Table 9: HRL levels with respect to weight ................................................................................... 12
Table of Figures
Figure 1: Strontium occurrence projection in the U.S. (from Skougstadt and Horr 1960) ............ 6
Figure 2: Frequency of strontium (primarily Sr-90) at USEPA National Priorities List sites. .......... 7
Figure 3: Strontium measured at facilities, averaged by zip code (NIRS 2003). (a) All reported
measurements, (b) measurements greater than 1.7 mg/L. ............................................... 9
1. Introduction
The purpose of this briefing paper is to provide background information on strontium, as
strontium is on the short list of contaminants being considered and evaluated further as part of the third
regulatory determination from the United States Environmental Protection Agency’s (USEPA) third
contaminant candidate list (CCL3) (Roberson 2012). The CCL3 listed 116 contaminants currently not
subject to any national primary drinking water regulation under the Safe Drinking Water Act (SDWA).
Because strontium is naturally occurring in the environment, the occurrence, health effects, analytical
methods, and treatment for strontium are important for the drinking water community to understand
prior to a regulatory determination and/or a potential regulation.
At a June 16, 2011 stakeholder meeting, the USEPA summarized occurrence data for 32 of the
116 CCL3 contaminants, including strontium (Table 1). Most of the national occurrence data used to
make regulatory determinations comes from prior Unregulated Contaminant Monitoring Rules (UCMRs).
Strontium was included in the earlier National Inorganics and Radionuclides Survey (NIRS) and is one of
the chemicals included in the ongoing Third Unregulated Contaminant Monitoring Rule (UCMR3)
monitoring.
Table 1: 32 CCL3 contaminants discussed during June 2011 stakeholder meeting (Roberson 2012)
32/116 CCL3 contaminants
Nitrosamines
Perfluorooctanic acid (PFOA)
N-nitrosodimethylamine (NDMA)
RDX (cyclotrimethylenetrinitramine)
N-nitrosodiethylamine (NDEA)
Dimethoate
N-nitrosodi-n-propylamine (NDPA)
Disulfoton
N-nitrosopyrrolidine (NPYR)
Diuron
N-nitrosodiphenylamine (NDPhA)
Molinate
Chlorate
Terbufos
Molybdenum
Terbufos sulfone
Strontium
Acetochlor
Vanadium
Acetochlor ethane sulfonic acid
1,1,2-Tetrachloroethane
Acetochlor oxanilic acid
1,2,3-Trichloropropane (TCP)
Alachlor ethane sulfonic acid
1,3-Dinitrobenzene
Alachlor oxanilic acid
1,4-Dioxane
Metolachlor
Methyl tert butyl ether (MTBE)
Metolachlor ethane sulfonic acid
1,3-Nitrobenzene
Metolachlor oxanilic acid
Perfluorooctane sulfonic acid (PFOS)
As part of the regulatory development process, health effects data are assessed in order to develop a
health reference level (HRL) to provide a benchmark for the occurrence data. The national occurrence
data for strontium, compared to the HRL, is summarized in Table 2.
2
Table 2: Summary of occurrence data for four CCL3 contaminants (adapted from Roberson 2012)
HRL
Occurrence
Systems or samples with detects
Contaminant
µg/L
Data Source
(>HRL, %)
Strontium*
4,200
NIRS
23/989 (2.33)
*Included in UCMR3; NIRS—National Inorganics and Radionuclides Survey
The Health Reference Level (HRL) for strontium is currently set at 4.2 mg/L and, according to the NIRS
database, 23 systems (2.3 %) contain samples with detects greater than the HRL level.
1.1 Physiochemical properties
The physiochemical properties of strontium are tabulated in Table 3. Strontium can exist in a 0
or +2 oxidation state, but in water and air strontium rapidly reacts and is only found in the Sr+2 form.
There are four stable strontium isotopes (84Sr, 86Sr, 87Sr, 88Sr).
Table 3: Physiochemical properties of strontium (The Merck Index 2006)
Property
Strontium
Molecular formula
Sr
CAS Number
7440-24-6
88 (82.6%)
Naturally occurring, stable isotopes
86 (9.86%)
(relative abundance)
87 (7.00%)
84 (0.56%)
Silvery-white metal,
Properties at room temperature
Face-centered cubic structure
Strontium accounts for 0.02-0.03% of the Earth’s crust, where it exists as strontium sulfate (celestite) or
strontium carbonate (strontianite). Strontium is used in many manufacturing processes as strontium
carbonate and strontium chloride. Table 4 provides additional chemical properties of strontium and
some common strontium compounds.
3
Table 4: Chemical identification of strontium and strontium compounds (ATSDR 2004)
Strontium
Strontium
carbonate
Strontium
chloride
Strontium
sulfate
7440-24-6
1633-05-2
10476-85-4
7759-02-6
Synonyms
---
Carbonic acid,
strontianite
Strontium
dichloride
Celestine;
celestite
Chemical Formula
Sr
SrCO3
SrCl2
SrSO4
Chemical structure
Sr
No data
Decomposes at
1100
875
1605
1250
No data
CAS Number
o
Melting point ( C)
o
Boiling point ( C)
Solubility in water (g/L)
777
1382
o
Decomposes
0.11 at 18 C
o
538 at 20 C
o
0.14 at 30 C
Aside from the stable isotopes, strontium has twenty-two radioactive isotopes most of which
have a half-life of less than 1 hour. The longest lived radioactive isotopes are listed in Table 5 and
persist for days except for Sr-90 which has a half-life of approximately 29 years. 89Sr and 90Sr are
formed during nuclear reactor operations and explosions (WHO 2010) making it likely to observe 90Sr
near Department of Energy sites in the United States.
Table 5: Radioactive strontium isotopes
Sr Isotope
Half-life
82
25.36 days
85
64.85 days
89
50.57 days
90
28.90 years
1.2 General uses and environmental exposure
Geologic weathering leads to naturally-occurring strontium in the environment. Strontium
occurrence is also linked to anthropogenic sources such as air contamination from milling processes,
coal burning, and phosphate fertilizers. Most of the air contaminants are eventually deposited on land
and soil. After the Chernobyl accident in the Ukraine, approximately 96% of radioactive 90Sr released
was wet deposited back on the Earth’s surface (WHO 2010).
In 1942, strontium (in the form of strontium sulfate), was stockpiled for the production of
4
strontium compounds used in defense during World War II. In 1963, the stockpile was gradually sold
and minimized (Ober 2002). Commercially, strontium is most commonly used as strontium sulfate and
strontium carbonate. Since 1970, most of the strontium consumed in the U.S. has been in the
manufacturing of ceramics and glass products (Ober 2002). In the U.S., strontium is required in the
faceplate glass of all televisions and color electronics containing cathode-ray tubes (CRT) to block X-ray
emissions. As CRT televisions have decreased in popularity with the rise of flat-panel technology not
requiring strontium faceplates, the use of strontium in the U.S. has also decreased (Ober and Angulo
2008). In 2001, strontium use in the U.S. was still primarily in the manufacturing of faceplate glass for
CRTs (WHO 2010).
Table 6: U.S. distribution of strontium compounds by end of use (by percent)
End use
2000
2001
Electrolytic production of zinc
2
2
Ferrite ceramic magnets
8
9
Pigments and fillers
2
3
Pyrotechnics and signals
9
9
Television picture tubes
77
75
Other
2
2
Total
100%
100%
It is often difficult to parse out non-radioactive from radioactive strontium occurrence in the
literature. The remainder of this paper will pertain to non-radioactive strontium unless explicitly labeled
as a discussion on radioactive strontium.
2. Occurrence
2.1 Environmental Occurrence
Naturally-occurring strontium is released into fresh water from geologic weathering associated
with sedimentary rocks such as gypsum, anhydrite, rock salt, limestone, and dolomite, as well as shales
and sandstones. While strontium mines have not been active in the U.S. since World War II, deposits in
Texas and California were the target of most mining efforts. Once in the environment, strontium forms
a hydration shell and is coordinated with six or more water molecules. Strontium ions sorb to clay
minerals, silicas, and iron oxides as hydrated ions. Carbonate enhances the sorption of strontium on
iron oxides. Strontium can accumulate in plants and organisms through uptake and bioaccumulation,
thus entering the human food chain.
5
Air
Strontium is released into the air through several anthropogenic routes including milling and
processing of strontium, burning coal, and land application of phosphate fertilizers. Most strontium in
the air is stable strontium (non-radioactive). Deposition of strontium is greatest near these
manufacturing, industrial, and agricultural activities (ATSDR 2004). Strontium concentrations
surrounding coal burning depends on the original concentration of strontium in the coal, amount of coal
burned, and the efficiency of fly ash recovery. Average concentrations of strontium in air are typically
below 0.1 µg/m3 (WHO 2010), but can vary greatly nearby coal burning plants.
Water
Strontium is present in almost all drinking water sources across the United States at an average
concentration range of 0.3-1.5 mg/L (WHO 2010; Skougstadt and Horr 1960) In water, most forms of
strontium are dissolved. A survey conducted by the U.S. Geological Society sampled 65 surface waters
across the U.S. to develop the occurrence map in Figure 1. This study identified the areas of northern
and western Texas and southern New Mexico and Arizona as those with the highest concentrations of
strontium. The other regions contained strontium measurements of less than 1.5 mg/L.
Figure 1: Strontium occurrence projection in the U.S. (from Skougstadt and Horr 1960)
Soil
Strontium is found in almost all rocks and soils in the United States. The worldwide average of
strontium concentration in soils is approximately 240 mg/kg. One non-natural release route of
6
strontium into soil is from the use of phosphate fertilizers (ATSDR 2004). Deposits in soil can also occur
as aerosol contamination (dust and particles containing strontium) which is subsequently wet deposited
as a result of rain; therefore, the deposit of strontium dust and particulate sources varies throughout
the U.S. and depends primarily on rainfall.
Radioactive Strontium
Strontium isotopes other than strontium-90 have a half-life so small that the potential hazard to
life is almost negligible. The following discussion of radioactive strontium only pertains to strontium-90
unless otherwise indicated.
Air: Radioactive strontium was released into the air above nuclear weapons testing sites from 19451980. Radioactive strontium releases from nuclear power plants in 1993 included 28 locations receiving
releases from boiling water reactors and 45 locations from pressurized water reactors (ATSDR 2004).
Water: Other than natural releases by geologic weathering, radioactive strontium was released
(intentionally and unintentionally) into freshwater sources as radioactive waste at U.S. Department of
Energy (DOE) sites. The release of strontium at the Savannah River Site, for example, was in conjunction
with basin purges in the nuclear reactor areas (Carlton et al. 1999). In 1993, 207.1 mCi 89Sr, 182.8 mCi
90
Sr, and 0.04 mCi 91Sr were cumulatively released into the environment from nuclear power plants
across the United States (ATSDR 2004). Nationwide, most 90Sr in water is from air particles depositing
into water sources.
Primarily 90Sr has been detected at hazardous waste sites throughout the United States. These
sites are part of the USEPA National Priorities List (NPL), a priority list of known releases of hazardous
substances across the U.S. The frequency of sites containing an active or pending citation for strontium
is depicted in Figure 2.
Figure 2: Frequency of strontium (primarily Sr-90) at USEPA National Priorities List sites.
7
Soil: The release of radioactive strontium has occurred at several DOE sites across the country, as
outlined above. From 1954-1989, 105 Ci of 89Sr and 299 Ci 90Sr were released into onsite seepage basins
at the Savannah River Site (ATSDR 2004). Many of the NPL hazardous wastes sites contain strontium in
the soil and sediment media (ATSDR 2004).
2.2 NIRS Database
The National Inorganics and Radionuclides Survey (NIRS) Database collected data from 989
CWSs served by ground water sources sampled between 1984 and 1986. The CWSs were selected from
a variety of sizes in order to represent the size stratification of all CWSs. From this database, 918
systems measured values of strontium. The NIRS data available was not associated with specific CWSs,
rather, information pertaining to the city and zip code of the sample was included in the NIRS Database.
In areas where more than one sample was measured, the 95th percentile of all samples was associated
with the corresponding zip code. The strontium concentrations associated with the zip code was used
to visually map the strontium levels from the NIRS database (Figure 3).
8
(A) Strontium in the United States, all values
(B) Strontium in the United States, > 2.1 mg/L (1/2 HRL)
Figure 3: Strontium measured at facilities, averaged by zip code (NIRS 2003). (a) All reported
measurements, (b) measurements greater than 1.7 mg/L.
From Figure 3a, many of the concentrations reported in the NIRS database are less than 1.7 mg/L. To
highlight the distribution of higher concentration areas, samples with greater than 2.1 mg/L (1/2 of the
HRL) strontium are only plotted in Figure 3b. The occurrence of strontium at facilities (a groundwater
study) does not directly correlate with the USGS map (a surface water study) (Figure 1) created in 1960.
9
From the NIRS dataset, 2.5 % (23) of the sampled locations contained strontium concentrations greater
than 4.2 mg/L and 5.2% (48) were above 2.1 mg/L. Both the 1960s occurrence dataset (surface water
only) and the NIRS dataset (groundwater only) are too small to confidently determine national
occurrence. A larger database would be needed as part of the regulatory development process to
understand the occurrence of strontium in drinking water across the United States. The ongoing UCMR3
monitoring will provide such a database.
2.3 Ongoing UCMR3 monitoring
The ongoing UCMR3 monitoring will provide a larger database to assist in the national
occurrence of strontium. Early UCMR3 data from USEPA’s first public release was compiled in Table 7
for over 1,400 reporting utilities, resulting in ~11,700 samples (USEPA, 2013). The data, thus far,
indicates that 0.5% of the samples are greater than the USEPA HRL of 4,200 µg/L.
Table 7: Preliminary strontium UCMR3 monitoring results—11,788 samples from approximately 1,400
utilities (USEPA, 2013)
Strontium concentration
Sample Statistic
(µg/L)
th
95 percentile
1,300
th
75 percentile
420
th
50 percentile
160
Maximum
55,100
3. Health Effects
Primary routes of exposure include inhaling aerosols and ingestion of food and water containing
strontium. In 1994, a diet study in the United Kingdom concluded that the total dietary exposure to
stable strontium was approximately 1.3 mg/day (Ysart et al. 1999). The distribution of strontium within
the body can vary between the genders (WHO 2010).
As a molecular surrogate for calcium, strontium has been administered to osteoporosis patients
at low doses over several years along with calcium and vitamin D with no adverse side-effects (ATSDR
2004). Strontium has the greater potential to accumulate in the bones of children than in adults
because of the high calcium requirement during the developmental stages of life. A child, therefore, is
more vulnerable to excess strontium because of the bone remodeling that occurs as a skeleton matures.
In rats, strontium bonded directly to the hydroxyapatite crystals, interfering with normal crystalline
growth (WHO 2010).
10
4. Technology Assessment
4.1 Analytical Methods
Several methods are approved for the measurement and analysis of strontium. Strontium was
included in the List 1 assessment monitoring for UCMR3. The approved methods UCMR3 analysis of
metals includes EPA Method 200.8, SM 3125, and ASTM D5763-10 with a MRL of 0.3 µg/L.
Table 8: Sample preparation and analysis methods for strontium determination in water (ATSDR 2004)
Sample preparation
Analytical method
Reference
Acid digestion
Filtration, acid digestion,
matrix modifier addition
Spectrophotometric
measurement (total Sr)
AOAC (1990)
FAAS (ASTM Method
D3920)
ASTM 1999
USEPA Method 200.7;
Wet acid digestion*
ICP-AES
Standard Methods 2012
Acid Digestion
USEPA Method 200.8 UCMR3 (2012),
Standard Methods (2012)
ICP-MS
*samples can also be analyzed without digestion when the turbidity is <1 NTU. However, UCMR3 requires
digestion for all samples for metals.
4.2 Treatment Technologies
Few treatment options are available for strontium. A study conducted in 1954 compared
strontium removal by chemical treatment used at treatment plants (Alexander et al. 1954). The study
determined that lime softening removed, on average, 73% of the strontium while a plant solely
practicing alum or ferrous sulfate coagulation would only see a 12% reduction. These removals are
dependent on higher calcium content. After the Fukushima Daiichi nuclear accident in early 2011,
several new articles were published with innovated treatment solutions for Sr-90. The focus on
radioactive strontium has been on removal of the radionuclides from the environment rather than on
treatment at a drinking water facility (Shimura et al. 2012).
5. Health Levels of Potential Interest
Strontium is considered an USEPA Cancer Class D (not classifiable as to human carcinogenicity)
(USEPA, 2012). There are several different health-associated levels linked to strontium consumption
and the USEPA uses the RfD to establish the maximum contaminant limit goal (MCLG) (EPA 822-R-03008 2003). The USEPA reference dose (RfD) is 0.6 mg/kg/day based on the NOAEL of 190 mg/kg/day to
prevent rachitic bone. Based on this RfD, an uncertainty factor (UF) of 300, and MF of 1
11
(http://www.epa.gov/iris/subst/0550.htm), a proposed MCLG or health recommended limit equals 4.4
mg/L using the default assumption of a relative source contribution of 20%. This value is in close
agreement with the health reference level of 4.2 mg/L strontium.
Equation 1
where:
NOAEL
LOAEL
BMD
UF
MF
=
=
=
=
=
no- or low-observed-adverse-effect level (mg/kg/day)
lowest-observed-adverse-effect level (mg/kg/day)
benchmark dose (mg/kg/day)
uncertainty factor
modifying factor
The MCLG is then calculated from the RfD according to Equation 6.
( )
Equation 2
where:
RfD
BW
RSC
I
=
=
=
=
Reference Dose
body weight (70 kg adults, 10 kg children, 4 kg infants)
Relative source contribution (fraction of RfD from drinking water)
daily intake of drinking water (2 liters for adults, 1 liter for children, 0.64
liter for infants)
The ability of strontium to bond directly to the hydroxyapatite and interfere with crystalline
growth could possibly create an increased vulnerability in children. Using the RfD of 0.6 mg/L and the
assumption of a relative source contribution (RSC) of 20% and an intake of 2 L/d regardless of age, HRLs
for strontium were calculated over a variety of weights (Table 9).
Table 9: HRL levels with respect to weight
Weight
(kg)
5
10
25
30
50
70
HRL
(mg/L)
0.32
0.63
1.58
1.90
3.17
4.43
12
Other than the above mentioned HRL value of 4.2 mg/L, several other values recommended for
reference doses, health reference levels, and maximum concentrations are in the literature. Below is a
summary list of several other limits recommended for strontium:



ATSDR has a maximum recommended limit based on an oral intermediate duration (15-364
days) of 2 mg/kg/day and an uncertainty factor or 30 (ATSDR 2013).
WHO has a tolerable daily intake: 0.13 mg/kg/day (WHO 2010)
EPA has three health advisory levels: a Lifetime level (HAL), a single-dose level (One day HAL),
and a 10-day HAL (10-day HAL). (WI DHS 2012)
o Lifetime HAL = 4 mg/L: a person drinks this level (or lower) their entire life and is not
expected to develop any health problems related to strontium exposure
o One-Day HAL = 25 mg/L: The level for which a child, drinking 1 L of water in a day, would
not be expected to develop any health problems related to strontium exposure.
o Ten-Day HAL = 25 mg/L: The level for which a child, drinking 1 L of water per day for ten
days, would not be expected to develop any health problems related to strontium
exposure.
6. Concluding Remarks
As of March, 2014, it is not completely clear how USEPA will handle strontium in its preliminary
third regulatory determination that is expected to be published later in 2014. However, strontium
should at least be considered as a potential positive regulatory determination, i.e., a national regulation
will eventually be developed. However, given the relatively low occurrence of strontium in the initial
UCMR3 monitoring results, a negative determination is also possible.
The timing for such a national regulation is also not completely clear. The third regulatory
determination would have to be finalized (maybe late 2015 or early 2016). Then, USEPA would have 24
months to publish a proposed regulation (late 2017 or early 2018), and 12 months thereafter to publish
a final regulation (late 2018 or early 2019). Water systems would have the typical three years to come
into compliance with the final regulation.
It is also not clear what the final Maximum Contaminant Level (MCL) might be. While the
current strontium HRL of 4,000 µg/L is certainly some type of current benchmark, establishing an MCL is
complex process that takes into account several additional benefit-cost considerations. So even with
the uncertainties about schedule and whether USEPA is going to regulate (or not regulate) strontium,
water systems may want to become aware of the potential regulatory implications for strontium for
their system.
7. References
Alexander, G.V., Nusbaum, R.E., MacDonald, N.S. (1954). Strontium and calcium in municipal water
supplies. Journal American Water Works Association, 46(7): 643-654.
ATSDR NPL Data. (2009). ATSDR webmapping platform. Accessed April 2013.
13
http://www.atsdr.cdc.gov/webmaps/datasources.asp
ATSDR. 2013. Minimal risk levels. 14 pages. Available at:
http://www.atsdr.cdc.gov/mrls/pdfs/atsdr_mrls_july_2013.pdf
Carlton, W.H., Simpkins, A.A., and Jannik, G.T. (1999). Radionulides in the Savannah River Site
environment. Document WSRC-MS-99-00667.
Ober, J.A. and Angulo, M.A. 2008. Strontium. In Minerals Yearbook: Metals and materials 2008. USGS
EdsVol 1.
Roberson, J.A. (2012). Informing regulatory decisions using national occurrence data. Journal of
American Water Works Association. 104 (3): E194-E203.
http://dx.doi.org/10.5942/jawwa.2012.104.0036.
Shimura H, Itoh K, Sugiyama A, Ichijo S, Ichijo M, et al. (2012) Absorption of Radionuclides from the
Fukushima Nuclear Accident by a Novel Algal Strain. PLoS ONE, 7(9): e44200.
http://dx.doi.org/10.1371/journal.pone.0044200.
Skougstads, M.W. and Horr, C.A. (1960). Occurrence of strontium in natural water. Geological Survey
circular 420.
The Merck Index. (2006). The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals.
Fourteenth Edition, Maryadele J. O’Neil, Patricia E. Heckelman, Cherie B. Koch, Kristin J. Roman,
Eds. (Merck & Co., Inc., Whitehouse Station, NJ, USA, 2006).
USEPA. (2003). Six Year Review chemical contaminants health effects technical support document. EPA
822-R-03-008 2003
USEPA (April 2012). 2012 Edition of the Drinking Water Standards and Health Advisories. EPA 822-S-12001. Available at:
http://water.epa.gov/action/advisories/drinking/upload/dwstandards2012.pdf
USEPA. (2013). The Third Unregulated Contaminant Monitoring Rule (UCMR 2): Data Summary
http://water.epa.gov/lawsregs/rulesregs/sdwa/ucmr/upload/epa815s14001.pdf. Last Updated
January 2014.
WHO. 2010. Concise International Chemical Assessment Document 77: Strontium and strontium
compounds. World Health Organization: England.
WI DHS (Wisconsin Department of Health Services). (2012). Strontium. Web access:
http://www.dhs.wisconsin.gov/eh/HlthHaz/fs/strontium.htm (30 March 2013).
14
Government Affairs Office
1300 Eye St. NW, Suite 701W
Washington, DC 20005
T: 202.628.8303
F: 202.628.2846
15