Citizen volunteers from the Carlsbad, New Mexico area were monitored for internally deposited radionuclides through a project entitled "Lie Down and Be Counted" (LDBC). This project is provided as an outreach service to the public to support education about naturally occurring and man-made radioactivity present in people who live in the vicinity of the WIPP. The data collected prior to the opening of the WIPP facility (26 March 1999) serve as a baseline for comparisons with periodic follow-up measurements that are slated to continue throughout the 35-year operational phase of the WIPP. It is important to note that these data represent an interim summary (through 1 October 1999) of an ongoing study.

Participating in the LDBC consists of a lung and whole body count every two years. Volunteers are recruited through presentations to local community groups and businesses. The entire measurement process takes approximately one hour. A detailed description of the measurement protocol, analysis, instrument and detection limits is provided in the CEMRC 1998 Report. In addition, the status of project and results are posted monthly on the CEMRC website and semi-annually through a newsletter distributed throughout the local community.

Results

As of 1 October 1999, 412 individuals have participated in the LDBC project. At the time the WIPP opened, 367 individuals had been measured using the in vivo protocol. This group of 367 measurements constitutes the pre-operational baseline to which subsequent results are compared. Counts performed after the opening of the WIPP are considered to be a part of the operational monitoring phase of the WIPP EM. Recounts of the original cohort began in July 1999, and 23 recounts had been performed through 1 October 1999.

Demographic characteristics (Table 21) of the current LDBC cohort are generally consistent with those reported in the CEMRC 1998 Report and the 1990 census for citizens living in Carlsbad. The largest deviations reported in the 1998 report between the LDBC cohort and 1990 census were the over-sampling of males and under-sampling of Hispanics. Since that time, improvements have been made with respect to the percent of males sampled, but Hispanics are still over 50% under-sampled relative to the 1990 census. In the future, demographic characteristics of the LDBC project will be compared to results of the 2000 census, which will be a more accurate representation of the current population. In addition, it is important to note that if the presence of a radionuclide is dependent on a subclass of interest (gender, ethnicity, etc.), valid population estimates can still be made by correcting for the proportion of under- or over-sampling for the particular subclass.

As discussed in detail in the CEMRC 1998 Report, the criterion, L_C, was used to evaluate whether a result exceeds background, and the use of this criterion will result in a statistically inherent 5% false positive error rate per pair-wise comparison (5% of all measurements will be determined to be positive when there is no activity present in the person). For the baseline measurements (n = 367), the percentage of results greater than L_C were consistent with a 5% random false positive error rate, at the 95% confidence level (1 to 9%), for all radionuclides except ²³²Th via ²¹²Pb, ²³⁵U / ²²⁶Ra, ⁶⁰Co, ¹³⁷Cs, ⁴⁰K, ⁵⁴Mn, ²³²Th via ²²⁸Ac and ⁶⁵Zn (Table 22). As discussed in detail in the 1998 report, five of these (²³²Th via ²¹²Pb, ⁶⁰Co, ⁴⁰K, ⁵⁴Mn (²²⁸Ac interference) and ²³²Th via ²²⁸Ac) are part of the shield-room background and positive detection is expected at low frequency. ⁴⁰K is a naturally occurring isotope of an essential biological element, so detection in all individuals is expected. ¹³⁷Cs and ²³⁵U / ²²⁶Ra are not components of the shielded room background and were observed at frequencies greater than the 95% confidence interval for the false positive error rate (discussed in more detail later). It is interesting that no result above L_C has been observed for ⁶⁵Zn. This finding may be a statistical anomaly, but may suggest an abnormality in the analysis algorithm at that photon energy.

For the operational monitoring counts (Table 22, n = 62), the percentage of results greater than L_C were consistent with baseline at a 95% confidence level (margin of error). The margin of error could not be calculated for many of the percentages because the sample size of the operational monitoring counts was too small at the time of this summary. When sufficient operational monitoring counts are performed, the margin of error will be calculated. In addition, it is important to note that operational monitoring includes the counting of new individuals and the recounting of previously measured participants. Based on the data reported herein, there is no evidence of a change in the frequency of detection of internally deposited radionuclides for citizens living within the vicinity of the WIPP since the WIPP began receipt of radioactive waste.

⁴⁰K results were positive for all participants (n = 412), ranging from 2120 to 6504 Bq per person with an overall mean (± SE) of 3959 (± 43) Bq. Such results are expected since K is an essential biological element contained primarily in muscle, and a theoretical constant fraction of all naturally occurring K is the radioactive isotope ⁴⁰K. The mean ⁴⁰K value for males (± SE), was 4531 (± 43) Bq, which was significantly greater (P < 0.0001) than that of females, which was 3218 (± 38) Bq. This result was expected since, in general, males tend to have larger body sizes and greater muscle content than females.

Detectable ¹³⁷Cs is present in 27.0 ± 4.4 % (95% confidence level, baseline and operational monitoring counts) of citizens living in the Carlsbad area. These results are consistent with preliminary findings reported in the CEMRC 1997 and 1998 reports. Detectable ¹³⁷Cs body burdens ranged from 5.6 to 62.9 Bq with an overall mean (± SE) of 10.8 (± 0.6) Bq. The mean ¹³⁷Cs body burden for males (± SE), was 11.6 (± 0.9) Bq, which was significantly greater (p = 0.01) than that of females, which were 9.0 (± 0.5 Bq).

Individual ¹³⁷Cs results were compared to two parameters of demographic data to determine whether the presence of ¹³⁷Cs was dependent on a particular demographic or lifestyle characteristic, using a Chi-square test of independence (Table 23). The presence of ¹³⁷Cs was independent of ethnicity, age, radiation work history, consumption of wild game, nuclear medical treatments and European travel. Occurrence of detectable ¹³⁷Cs was significantly associated (p = 0.003) with gender, where males had higher prevalence of ¹³⁷Cs relative to females. Presence of ¹³⁷Cs was also associated with smoking habit (where smokers had a higher prevalence relative to non-smokers), but at a lower significant level (p=0.032). The results reported herein are consistent with those reported in the CEMRC 1998 Report, except that gender association was not significant (p = 0.06) in the earlier subset of the baseline data. The frequency of smoking among males and females was statistically indistinguishable (17.8 % for males, 16.6 % for females, p = 0.807), eliminating a correlation between the two variables as a confounding factor. It is likely that the association with gender is related to the tendency for larger muscle mass in males than in females, as supported by the ⁴⁰K results. The association of ¹³⁷Cs with smoking habit could be related to the presence of fallout ¹³⁷Cs in tobacco, decreased pulmonary clearing capability in smokers, or other as yet unidentified factors, and further study is warranted.

As reported in the CEMRC 1998 Report, the percentage of results greater than L_C for ²³⁵U/²²⁶Ra (11.1%) are significantly (although slightly) higher than the distribution-free confidence interval for a 5% random false positive error rate (1 to 9%). These data are not nearly as compelling as those for ¹³⁷Cs, but the large sample size of the current cohort tends to support the observed pattern. Although ²³⁵U and ²²⁶Ra cannot be differentiated via gamma spectroscopy, it is likely the signal is the result of ²²⁶Ra because the natural abundance of ²²⁶Ra is much greater than that of ²³⁵U. However, further study, possibly involving in vitro bioassay for the two radionuclides (the nuclides can be distinguished via alpha spectrometry), is needed.