Introduction
Stable iodine prophylaxis is an important measure against thyroid cancer after nuclear emergencies, alongside sheltering, evacuation, and avoiding the consumption of contaminated food and beverages. 1 , Reference Leung, Bauer and Benvenga 2 Intake of stable iodine can saturate the thyroid gland, block the radioactive iodine, and thus reduce the internal radiation exposure. 1 Pharmacological information and distribution method should be clearly conveyed to appropriately implement stable iodine among children and pregnant women.Reference Nishikawa, Kohno and Takahashi 3
Knowledge of the benefits and harms of stable iodine, methods of internal administration, and methods of distribution were factors associated with the intake of stable iodine.Reference Nishikawa, Kohno and Takahashi 3 In times of disasters, as the time for decision-making is limited, it would be important to provide information in normal times. As stable iodine implementation requires oral administration, it has been reported that careful communication would be important. Awareness and risk perception of stable iodine in several areas near nuclear power plants have been reported.Reference Yamada, Orita and Matsunaga 4 –Reference Matsunaga, Orita and Taira 6 Still, there was little information about stable iodine knowledge in actually implemented settings.
The Great East Japan Earthquake and associated tsunami struck northeastern Japan on March 11, 2011, triggering the Fukushima Nuclear Disaster. Radioactive materials were released into the environment, resulting in unintended population exposure.Reference Tsubokura, Gilmour and Takahashi 7 Estimated thyroid absorbed doses for most residents remained below 100 mGy, 8 a level at which stable iodine prophylaxis was not necessarily indicated under the guidance. 9 Nevertheless, seven municipalities distributed stable iodine tablets and four issued formal instructions for their use. 10 In addition to the government-led Fukushima Health Management Survey,Reference Yasumura and Abe 11 the Research Institute of Radiation Safety for Disaster Recovery Support (RSDRS) has maintained a community-based voluntary thyroid screening program since 2012Reference Watanobe, Furutani and Nihei 12 –Reference Nishikawa, Oguro and Suzuki 15 in cooperation with municipalities inside and outside the Fukushima Prefecture. The program has been collecting clinical data and questionnaire-based information on whether stable iodine was taken at the time of the disaster.
The present study examined stable iodine knowledge and information-provider preferences among parents attending RSDRS screenings. It would be important to provide health information on thyroid cancer prevention during nuclear emergencies, and this study helps the local or national authorities to clarify who was not well informed and how the information on stable iodine should be communicated.
Methods
Study Design, Setting, and Participants
The present study was a cross-sectional descriptive study using self-administered questionnaires distributed before thyroid ultrasound examinations at RSDRS in Fukushima Prefecture between April 2019 and December 2022. The study was conducted among parents of participants in thyroid ultrasound examination programs established as post-accident health surveillance in Fukushima Prefecture. Data from the last examination round were extracted. The examination program serves residents from multiple municipalities with varying stable iodine responses. Some municipalities have implemented pre-distribution programs, others maintain emergency stockpiles, and some lack formal distribution systems due to the distance.Reference Nishikawa, Kohno and Takahashi 3
Thyroid screening at the RSDRS
RSDRS has offered voluntary thyroid screening since 2012, operating independently of the Fukushima Health Management Survey (FHMS). The programs differ in two principal respects. In terms of eligibility, FHMS is restricted to Fukushima Prefecture residents, whereas RSDRS accepts participants from both within and outside the prefecture, including those from neighboring areas such as Ibaraki Prefecture.
Measurements and Variables
Self-administered questionnaire
Prior to ultrasonography, parents were invited to complete a paper-based questionnaire. Beyond the items described below, the questionnaire captured whether they had taken stable iodine orally at the time of the Fukushima Nuclear Disaster. For school-based screening, questionnaires were distributed to families in advance; parents completed them at home and returned them on the day of the school examination. All forms were centrally archived at RSDRS.
Knowledge assessment
This questionnaire was developed by YN, CH, and MT based on previous research and guideline. 1 , Reference Yamada, Orita and Matsunaga 4 Prior to its use, it was reviewed by RSDRS staff as well as by staff members who had participated as representatives of local municipalities. Since asking numerous questions of disaster-affected individuals would impose an excessive burden, the survey items were limited to the following questions, which were acceptably incorporated as part of routine clinical inquiries. Knowledge was assessed using 3 items addressing key domains of stable iodine prophylaxis:
1. Role recognition: “Do you know the role of stable iodine tablets?”
Role recognition was assessed by asking whether respondents were aware of the role of stable iodine tablets (Yes/No), with “Yes” scored as 1 (correct) and “No” as 0.
2. Distribution method awareness: “How are stable iodine tablets distributed in your area?”
Distribution method awareness was assessed by asking how stable iodine tablets are distributed in the respondent’s area, with four response options: pre-distribution, stockpiled for emergency distribution, not distributed in the area, or don’t know. Respondents who selected any specific distribution method were scored as 1, indicating awareness of distribution mechanisms in their area, while those who responded “don’t know” were scored as 0. Since participants came from multiple municipalities with varying stable iodine policies, all three specific distribution responses were considered valid indicators of distribution system awareness; the “don’t know” response was treated as a fundamental lack of knowledge about distribution mechanisms, regardless of local policy. This scoring approach focused on whether respondents possess any knowledge of distribution systems (even if identifying absence of distribution in their area) versus having no knowledge of how stable iodine might be accessed.
3. Effectiveness duration: “How long is stable iodine effective in preventing thyroid exposure?”
Effectiveness duration was assessed by asking how long a dose of stable iodine remains protective against thyroid irradiation; response choices were 24 hours, 3 days, 7 days, or don’t know. Only 24 hours, the pharmacologically accurate answer, was scored 1.
Then, a total score (0-3) was computed by summing the three item scores.
Preferred information providers
“If a nuclear emergency occurs in the future, from whom would you like to receive information about stable iodine tablets?”
Response options were developed to reflect information providers relevant to Japan’s nuclear emergency preparedness framework.Reference Willems, Carr and Dreger 16 , 17 Response options included: Health professionals/Government officials/School teachers/Neighbors. Multiple selections were permitted to capture all preferred information providers.
Additional variables
Age at examination, sex, and test date, children’s iodine intake, and their parents’ iodine intake were extracted from examination records. Years since the disaster (March 11, 2011) were calculated from test dates as continuous variables and categorized into periods (0-4 years, 5-9 years, 10+ years post-disaster).
Analytical Methods
Categorical data are presented as frequencies and percentages; continuous variables are summarized using means, standard deviations, medians, and interquartile ranges. Variation in knowledge scores across information-provider preference groups was explored by calculating group means, standard deviations, and mean differences. These comparisons are presented as descriptive characterizations of the data rather than hypothesis tests, and no causal inferences are drawn. We examined temporal patterns using Spearman’s correlation between years since disaster and knowledge scores. Given the cross-sectional design, potential for selection effects, and small magnitude of observed differences, all comparative statistics should be interpreted cautiously as descriptive characterizations. Statistical analyses were conducted using R version 4.4.3.
Ethical Considerations
This study was approved by the ethics board of Hirata Central Hospital (2017-0321-2), the Ethics Committee of the Kyoto University Graduate School of Medicine (R1459), and the Fukushima Medical University Certified Review Board (30180). Formal informed consent was obtained from all participants (their parents in the case of participants aged <20 at the time of the screening) included in this study.
Results
Participant Characteristics
The study included 1,116 participants (Table 1) with a mean age of 12.3 years (SD = 1.9; median = 13; IQR 11-14; range 1-18). Most participants (85.8%, n = 957) were aged 10-14 years, with a balanced gender distribution (47.0% female). Examinations occurred 8.1-11.8 years post-disaster (mean = 9.9 years, SD = 1.5), with 56.7% examined during years 5-9 and 43.3% during year 10 or beyond.
Characteristics of parent respondents and their children
Table 1. Long description
1,116 parents/children. Children: mean age 12.3y (SD 1.9), median 13 (IQR 11-14); age groups: 0-9y n=84 (7.5%), 10-14y n=957 (85.8%), 15-19y n=75 (6.7%); sex: female 525 (47.0%), male 591 (53.0%). Years since disaster: mean 9.9y (SD 1.5), range 8.1-11.8; 5-9y post-disaster n=633 (56.7%), 10+y n=483 (43.3%). Child’s stable iodine intake (n=1080): yes 413 (38.2%), no 546 (50.6%), not yet born 121 (11.2%); 36 missing. Parent’s intake (n=1009): yes 419 (41.5%), no 590 (58.5%); 107 missing. “No” includes those not designated as recipients of stable iodine.
Stable iodine intake at the time of the 2011 Fukushima nuclear disaster. Stable iodine “No” intake includes those who reported not taking iodine or were not designated as recipients of stable iodine. 95% CI = 95% confidence interval. 95% CIs were calculated among respondents with available data.
Regarding stable iodine intake after the Fukushima Nuclear Disaster, data were available for 1,080 children (96.8%) and 1,009 parents (90.4%). Among children with available data, 38.2% (n = 413) had taken stable iodine, 50.6% (n = 546) had not, and 11.2% (n = 121) had not yet been born at the time of the disaster. Among parents with available data, 41.5% (n = 419) had taken stable iodine and 58.5% (n = 590) had not.
Overall Knowledge Levels Among Parents
The mean parental knowledge score was 1.66/3 (SD = 0.97; median = 2, IQR 1-2) (Table 2). Knowledge level distribution was: High (score 3), 22.1% (n = 237/1,070); Moderate (score 2), 35.7% (n = 382/1,070); Low (score 0-1), 42.1% (n = 451/1,070).
Knowledge of stable iodine prophylaxis among parent respondents
Table 2. Long description
1,070/1,116 valid scores. Overall knowledge score (0-3): mean 1.66 (SD 0.97), median 2 (IQR 1-2). Item 1, Role recognition (n=1107): Yes/correct 750 (67.8%), No 357 (32.2%). Item 2, Distribution method awareness (n=1109): aware 507 (45.7%), not aware 602 (54.3%). Among aware: stockpiled 360 (71.0%), not distributed in area 73 (14.4%), pre-distribution 74 (14.6%). Item 3, Effectiveness duration (n=1079): correct (24 hours) 552 (51.2%). Among 527 incorrect: believed 3 days 348 (66.0%), believed 7 days 173 (32.8%); 6 excluded (invalid free-text responses).
95% CI = 95% confidence interval. *Six respondents provided incorrect free-text responses and were excluded from subgroup analysis.
Knowledge by Individual Items
1. Role recognition
Two-thirds (67.8%, n = 750/1,107) correctly answered “Yes” to knowing stable iodine’s role.
2. Distribution method awareness
This item showed the poorest performance. Among 1,109 responses, 54.3% (n = 602) lacked awareness of distribution methods (responding “don’t know”), while 45.7% (n = 507) demonstrated awareness of specific distribution methods in their area. Among aware respondents, the majority identified stockpiling (71.0%, n = 360), followed by pre-distribution (14.6%, n = 74) and not distributed in the area (14.4%, n = 73).
3. Effectiveness duration
Just over half (51.2%, n = 552/1,079) correctly identified 24 hours. Among those who responded incorrectly (n = 527), 66.0% (n = 348) believed 3 days and 32.8% (n = 173) believed 7 days.
Preferred Information Providers
Health professionals were overwhelmingly preferred (90.5%, n = 1,010), followed by government officials (17.6%, n = 196), school teachers (10.6%, n = 118), and neighbors (0.5%, n = 6) (Table 3). Most respondents (81.8%, n = 913) selected only one information provider (mean = 1.19, SD = 0.49). The most common pattern was health professionals alone (73.7%, n = 823), followed by health professionals plus government officials (10.0%, n = 112).
Preferred information providers for future nuclear emergencies
Table 3. Long description
Preferred information providers for future nuclear emergencies (multiple selections allowed). Single providers: health professionals 1010 (90.5%), government officials 196 (17.6%), school teachers 118 (10.6%), neighbors 6 (0.5%). Most common combinations: health professionals only 823 (73.7%), health professionals + government officials 112 (10.0%), government officials only 44 (3.9%), school teachers only 41 (3.7%). HP = Health Professionals.
HP = Health Professionals; multiple selections permitted. 95% CI = 95% confidence interval.
Mean knowledge scores varied modestly by provider preference (Table 4). Those preferring health professionals scored 1.67 versus 1.56 for non-preferers (difference = 0.11 points). Those preferring government officials scored 1.83 versus 1.62 (difference = 0.20 points, P = 0.007, reported for exploratory reference). Those preferring school teachers scored 1.51 versus 1.68 (difference = −0.17 points). Differences across groups were small in absolute terms (range 1.51 to 1.83, spanning 0.32 points on a 0-3 scale).
Knowledge scores by preferred information provider
Table 4. Long description
Mean knowledge scores (0-3 scale) compared by preference for each information provider, independent t-tests. Health professionals: preferred n=972, mean 1.67 (95%CI 1.61-1.73); not preferred n=98, mean 1.56 (95%CI 1.39-1.73); difference 0.11 (95%CI -0.07 to 0.29), p=0.233. Government officials: preferred n=192, mean 1.83 (95%CI 1.69-1.96); not preferred n=878, mean 1.62 (95%CI 1.56-1.69); difference 0.20 (95%CI 0.06-0.35), p=0.007. School teachers: preferred n=112, mean 1.51 (95%CI 1.32-1.70); not preferred n=958, mean 1.68 (95%CI 1.62-1.74); difference -0.17 (95%CI -0.37 to 0.03), p=0.102.
95% CI = 95% confidence interval. Mean (95% CI) from independent samples. P-values from t-tests.
Discussion
This descriptive study reveals a critical paradox in post-disaster nuclear preparedness: despite high awareness of stable iodine’s existence following the Fukushima accident, operational knowledge essential for actual protective action remains deficient nearly a decade later. This gap between conceptual awareness and actionable knowledge has important implications for disaster preparedness communication strategies.
Our findings showed a distinction between awareness and operational knowledge. While two-thirds recognized stable iodine’s protective role, over half could not identify how to access it during emergencies. This pattern suggested that awareness raising successfully introduced the concept; however, there was room for improvement for appropriate implementation of stable iodine. Previous studies near operating nuclear power plants showed similar patterns, high awareness coexisting with limited preparedness behaviors.Reference Matsunaga, Orita and Taira 5 , Reference Matsunaga, Orita and Taira 6 This finding across disaster-experienced and at-risk populations indicated that there would be room to be improved in risk communication approaches. Additionally, misconceptions about effectiveness duration might complicate this problem. Understanding stable iodine’s role, distribution methods, and effectiveness duration is essential for enabling timely and appropriate protective responses toward nuclear emergencies.
This finding has implications for the roles of each stakeholder in nuclear emergency preparedness. Health professionals were largely preferred in this study. It should be noted that “health professionals” as a response category can cover a broad range of providers, including physicians, nurses, public health nurses (hokenshi), and pharmacists may be particularly well positioned for preparedness communication from peacetime. The National Diet’s Fukushima Nuclear Accident Independent Investigation Commission report stated that “it is desirable for healthcare professionals, particularly pharmacists, to be present at evacuation shelters to assist with the distribution and administration of stable iodine.” 10 While preference of health care providers was not associated with knowledge level; due in part to not many people selecting the item, there was a modest association between government official preference and slightly higher knowledge. This may reflect that those actively seeking preparedness information recognize national/local government’s distribution role. 17 From this standpoint, public health nurses (hokenshi), most of whom work in the public sector as gatekeepers of community health in Japan, would also play an important role in Japan. As interprofessional collaboration has been shown to promote effective counseling and improve access to medications in other health care contexts,Reference Goto, Wirtz and Hayashi 18 similar coordinated efforts among health professionals, government officials, and community networks may be beneficial for stable iodine preparedness. Additionally, although neighbors were rarely preferred as knowledge sources, neighbors’ community networks can serve as effective distribution channels. In Miharu Town, a community-based approach utilizing 8 district networks achieved stable iodine delivery to 94.9% of target households.Reference Nishikawa, Kohno and Takahashi 3 This underscored the importance of recognizing that each stakeholder—health professionals, government officials, and community networks—contributes distinctly to preparedness, and that concerted efforts across these roles are essential for better access to stable iodine.
Implementation of stable iodine preparedness can be considered across 2 distinct timeframes, normal times and emergencies.Reference Reynolds and W Seeger 19 – 21 During normal times, building foundational knowledge, covering stable iodine’s role, effectiveness duration, and access methods, is essential. During emergencies, however, the knowledge required varies by scenario. In evacuation shelter settings, for instance, distribution is primarily managed by authorities at the site, making prior information on access logistics less critical for individuals, while core knowledge of appropriate timing and effectiveness duration remains universally necessary. In shelter-in-place scenarios, by contrast, awareness of local distribution mechanisms may become more important. These variations call for a layered information sharing approach, core pharmaceutical information applicable to all residents including normal times, complemented by specific instruction on access methods tailored to each municipality’s preparedness planning. In a situation where misinformation and disinformation about radiation risks can spread rapidly,Reference Tsubokura, Onoue and Torii 22 a multisectoral communication strategy is particularly valuable.Reference Borges do Nascimento, Pizarro and Almeida 23 Such an approach, government authorities, media coverage,Reference Lisus, Nishikawa and Saito 24 coordinating health professionals, and community networks, would ensure that accurate knowledge reaches the public and serve the distinct information needs of each timeframe.
There are several limitations. First, this cross-sectional design cannot establish causality or temporal sequences. Second, information source preferences reflect hypothetical future choices, not actual past information-seeking behaviors or sources. Third, this study was not able to collect sufficient information about the parents. Nevertheless, as a questionnaire incorporated into health examinations conducted for disaster-affected individuals, it was practically feasible in terms of both length and content, and has the potential to yield lessons for the future. In this regard, this study represented one example of a field study that can be integrated into post-disaster health care practice. Also, since this survey was conducted as part of the pre-examination questionnaire for thyroid screening, health care professionals may have been more readily imagined as information source providers. Finally, our surveillance program sample may not represent the broader population, potentially overestimating knowledge levels.
Conclusions
Despite high awareness of stable iodine following the Fukushima disaster, this study revealed critical operational knowledge gaps: 54% did not know how stable iodine could be distributed. Given their role as the most trusted information source, health professionals may play a key role in addressing these gaps by communicating specific preparedness information to communities in high-risk regions through targeted, multi-channel strategies.
Acknowledgment
We would like to thank all the staff who engaged in this screening at RSDRS and the municipality officers involved. The screening was conducted by RSDRS as a clinical/ public health practice, and all basic support was provided by RSDRS. YN, CS, FO, and MT have been working as medical doctors at Hirata Central Hospital, which is affiliated with RSDRS. YN was supported by JSPS KAKENHI Grant Number JP22K17367, JP26K20576, Network-type Joint Usage/Research Center for Radiation Disaster Medical Science Joint Research Projects, FY2025 MUSUBIME of Kyoto University, The Japan Medical Association, The Japan Cancer Society. Kurita Water and Environment Foundation, and Takemi Program in International Health, Harvard T.H. Chan School of Public Health. These organizations played no role in this study.
Author contribution
Conception and design (all authors); data collection (YN, CS, MT); data analysis (YN, MT); interpretation of the results (all authors); drafting of the article (YN, AG); critical revision of the article for important intellectual content (all authors); final approval of the article (all the authors).
Funding statement
This work was supported by the Program of the Network-type Joint Usage/Research Center for Radiation Disaster Medical Science.
Competing interests
Outside the submitted work, YN received personnel fees from Japan Association for Clinical Engineers, Japan Society for the Promotion of Science, and EUCALIA Inc., grants from Japan Cancer Society and Japan Medical Association, and a donation from Datack Inc.
AI statement
The authors used Anthropic Claude Sonnet 4.6 to improve the English language of this manuscript. The original text was written by the authors, who reviewed and edited the output. The authors take full responsibility for the content of this publication.
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