An unexpected rise in strontium-90 in US deciduous teeth in the 1990s

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Abstract

For several decades, the United States has been without an ongoing program measuring levels of fission products in the body. Strontium-90 (Sr-90) concentrations in 2089 deciduous (baby) teeth, mostly from persons living near nuclear power reactors, reveal that average levels rose 48.5% for persons born in the late 1990s compared to those born in the late 1980s. This trend represents the first sustained increase since the early 1960s, before atmospheric weapons tests were banned. The trend was consistent for each of the five states for which at least 130 teeth are available. The highest averages were found in southeastern Pennsylvania, and the lowest in California (San Francisco and Sacramento), neither of which is near an operating nuclear reactor. In each state studied, the average Sr-90 concentration is highest in counties situated closest to nuclear reactors. It is likely that, 40 years after large-scale atmospheric atomic bomb tests ended, much of the current in-body radioactivity represents nuclear reactor emissions.

Introduction

Since man-made fission products were first released into the environment in the mid-1940s, determining in vivo levels of these radioisotopes has challenged scientists. Hundreds of radioisotopes are created in nuclear weapon detonations and in nuclear reactor emissions. Many of these are short-lived, and therefore highly unlikely to track in vivo. Collecting samples of longer-lived isotopes often involves invasive processes such as autopsies and biopsies, making collection of significant samples time-consuming and costly.

In the US, whose government conducted 206 atmospheric tests of nuclear weapons from 1946 to 1962 (100 in Nevada, 106 in the South Pacific) (Norris and Cochran, 1994), the federal government instituted programs measuring strontium-90 (Sr-90) concentrations in vertebrae. One focused on deceased adults (begun 1954, 3 cities, ∼50 bones per year) (Klusek, 1984), while the other included deceased children and adolescents (begun 1962, 30 cities, ∼300 bones per year) (Baratta et al., 1970). Both showed increases to a peak in 1964, just after the Partial Test Ban Treaty was signed, and a dramatic decline in the mid- and late 1960s.

The largest-scale US program studying in-body radioactivity was conducted in St. Louis. Kalckar suggested that large numbers of deciduous teeth could be collected and tested to examine the buildup of fallout from bomb tests (Kalckar, 1958). A coalition of St. Louis medical/scientific professionals and citizens collected over 300 000 teeth from local children from 1958 to 1970. Results from St. Louis were similar to the two bone programs, i.e.

  • A 55-fold rise in average millibecquerels (mBq) of Sr-90 per gram calcium at birth (7.4–408.1) took place for 1949–1950 births (before large-scale tests began) to 1964 births (just after the largest-scale bomb tests ended).

  • A 50% decline in Sr-90 concentrations in St. Louis fetal mandibles occurred from 1964 to 1969 births. This far exceeded the expected 9% reduction suggested by the 28.7 year half-life of Sr-90 (Rosenthal, 1969).

After the bone and tooth studies showed such a rapid post-1964 decline, federal funding was terminated for each program, and work ceased. The tooth study ended in 1970, the child bone study in 1971 and the adult bone study in 1982.

The US studies were accompanied by similar international efforts. Each independently confirmed the American findings of rapid increases in teeth until 1964, including studies in Czechoslovakia, Denmark, Finland and Scotland (Santholzer and Knaifl, 1966, Aarkrog, 1968, Rytomaa, 1972, Fracassini, 2002). Another study in Finland duplicated the rapid post-1964 plunge in Sr-90 (Kohlehmainen and Rytomaa, 1975). No nation maintained an ongoing program, but after the Chernobyl accident, reports from Germany, the Ukraine and Greece documented a substantial rise in Sr-90 in baby teeth after the April 1986 disaster (Scholz, 1996, Kulev et al., 1994, Stamoulis et al., 1999). Another study examined Sr-90 in teeth from children who lived proximate to the Sellafield nuclear installation in northwestern England; results are addressed in Section 4 (O'Donnell et al., 1997).

With no program of in vivo radioactivity to gauge the burden on the body, levels in the environment can be used as a proxy measure. In the past, patterns of Sr-90 in baby teeth were roughly equivalent to those of Sr-90 in milk (Rosenthal et al., 1964). The US government (beginning 1957) began publicly reporting monthly levels of a variety of radionuclides in milk and water in 40–60 US locations. However, a number of these radioisotopes, including Sr-90, strontium-89, cesium-137, barium-140 and iodine-131 were discontinued in the early 1990s (National Air and Radiation Environmental Laboratory, 1975–2001).

One measure that is still publicly reported is the concentration of gross beta particles in precipitation. A reduction in average beta levels reversed after 1986–1989. While the most recent 4-year period still features incomplete data, thus far the increase from 1986–1989 to 1998–2001 has been 53.8%. This difference is significant at P<0.0001, i.e. the probability of this increase due to random chance is less than 1 in 10 000 (Table 1).

The lack of an ongoing program measuring in vivo radioactivity levels and an unexpected, sustained rise in environmental beta concentrations warrant a resumption of testing in vivo Sr-90 and perhaps other radioisotopes, first instituted during the era of atmospheric nuclear weapons testing.

In 1996, the Radiation and Public Health Project (RPHP) began a study of Sr-90 levels in deciduous teeth, focused on persons living near nuclear reactors. The goal of this project was to build a current database of in vivo radioactivity documenting Sr-90 patterns and trends. While Sr-90 is just one of hundreds of radioisotopes from fission, it can be used as a proxy for all fission products, especially those with extended half-lives.

Section snippets

Materials and methods

Earlier reports addressed methods used and initial findings from the baby tooth study (Gould et al., 2000a, Gould et al., 2000b, Mangano et al., 2000). These teeth were processed using a scintillation counter from the University of Waterloo in Ontario, Canada. In June of 2000, RPHP leased a Perkin-Elmer 1220-003 Quantulus Ultra Low-Level Liquid Scintillation Spectrometer. Introduced in 1995, only approximately 15–20 units are now in use in the US (Laxton, Mark, Perkin-Elmer Life Sciences Inc.,

By state

A total of 2089 teeth were tested for Sr-90, and are discussed in this paper (another 1335 had been tested previously using a different scintillation counter and method). As discussed, the two sets of results are each internally consistent, but not comparable with each other because of differences in the counter, its location, level of background radiation and method of cleaning teeth, so only the last 2089 teeth are used. Of these, 1592 (77%) were from children born in the five states

Discussion

The US has conducted no official program measuring in vivo levels of fission products for over 20 years. This report introduces current data on patterns and trends of Sr-90 concentration in US baby teeth, mostly near nuclear power installations. The average concentration of Sr-90 was 132 mBq Sr-90/g Ca for all children born after 1979, when in vivo Sr-90 remaining from atomic bomb tests should approach 0.5

Study limitations/opportunities for further study

This report represents the first large-scale study of US in vivo levels of radioactivity in several decades. Although the initial findings presented here are important ones, they raise various questions that should be addressed in future research.

Other unexplored factors may help explain the temporal trends affected here. For example, the current study collected auxiliary data on mother's age at delivery and source of drinking water. Analyzing results by basic characteristics such as gender and

Acknowledgements

Jerry Brown, Ph.D., is acknowledged for his contribution in collecting baby teeth in southeastern Florida.

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