Effect of Microgravity and Space Radiation Exposure on Human Oral Health: A Systematic Review

Abstract

A systematic review was conducted to assess the effects of microgravity and space radiation on astronauts’ oral health. This review aimed to determine if these conditions increase the risk of dental and periodontal diseases, identify pre-mission dental care strategies, and specify relevant dental emergencies for astronauts to manage during missions. Following PRISMA guidelines, the review was registered on PROSPERO (CRD42023472765). Databases including PubMed, Scopus, Web of Science, Cochrane Library, and OVID Medline were searched. Of the 13 studies identified, 7 were eligible for qualitative synthesis. The included studies revealed that space conditions compromise oral health. Findings indicate changes in saliva composition, with a significant decline in salivary lysozyme levels during missions lasting 28 to 84 days. Salivary IgA levels also increased before and peaked after flights (microgravity alters fluid shear and protein folding). Viral reactivation was a key finding, with latent viruses such as Epstein–Barr virus (EBV), cytomegalovirus (CMV), and varicella zoster virus (VZV) being reactivated during missions (immune suppression and gene expression shifts under spaceflight stress). Data from a study found that 50% of crew members shed viruses in their saliva or urine, and 38% tested positive for herpesviruses. The included studies also documented alterations in the oral microbiome, including increased gastrointestinal and decreased nasal microbial diversity. This suggests alterations in salivary biomarkers, viral shedding, and microbiome changes in astronauts during long-duration missions. These changes appear associated with immune dysregulation and stress, but causality remains uncertain due to observational designs, small heterogeneous samples, and confounding factors. Although current evidence is indicative rather than definitive, these findings highlight the need for preventive dental measures prior to missions and preparedness for managing oral emergencies in-flight. Future studies should address the mechanistic separation of microgravity and radiation effects, with implications for upcoming Moon and Mars missions.

1. Introduction

The exploration of space represents one of humanity’s most remarkable achievements, opening new frontiers in science, technology, and human capability [1]. As we continue to advance space exploration, it is crucial to address the health and well-being of astronauts—not only their overall physical health but also their oral health [2,3]. The unique space environment, characterized by microgravity and exposure to cosmic radiation, raises concerns regarding its potential effects on dental and periodontal health [4,5,6]. This review explores the significance of these concerns and presents focused research questions aimed at building a more robust evidence base.

Lloro V. [7] recently conducted a systematic review to assess the available evidence on the impact of space and simulated microgravity on oral health, with the goal of preventing dental complications during both short- and long-term space missions. The review included findings related to oral health alterations caused by microgravity and related conditions, also considering the duration of exposure. A total of 103 cases with diverse oral health issues were identified in the literature. These included a wide variety of physiological and dental changes experienced by astronauts. The findings were categorized based on mission duration [7].

Long-term space missions were associated with events such as dental pain and dental caries (each reported once during spaceflight), along with an increase in anaerobic and streptococcal species in saliva and dental plaque, observed in 12 cases during both actual missions and simulated conditions. Short-term exposure was linked to changes in salivary components and dental conditions, including elevated salivary IgA, crown displacement, increased pain in various oral regions, the presence of Mycoplasma, higher salivary IgM and IgG levels, delayed wound healing, increased alpha-amylase and cortisol concentrations, and upregulated salivary MMP8 and MMP9. These were all observed in simulation-based studies [7].

The conditions astronauts face in space, namely microgravity and exposure to cosmic radiation, are distinctly different from those on Earth [8]. Microgravity, a near-weightless state, induces a range of physiological adaptations, impacting not only the musculoskeletal and cardiovascular systems but potentially oral tissues as well. Space radiation further contributes to cellular and tissue damage, posing additional health risks. The combination of these factors presents unique challenges for maintaining systemic and oral health during missions [8,9,10].

Prevention remains a cornerstone of oral healthcare. This review explores strategies and recommendations for preserving astronauts’ oral health prior to mission launch. Implementing preventive measures may reduce the incidence of oral health issues during spaceflight [11,12].

In the remote and resource-limited setting of space, astronauts must also be equipped to manage medical emergencies, including dental issues. This raises practical questions about training and preparedness, particularly in the absence of immediate access to professional dental care.

The outcomes of this research have broader implications beyond astronaut health. Understanding the impact of microgravity and radiation on oral health may inform the development of targeted preventive and therapeutic strategies. Furthermore, these insights could benefit individuals in isolated or underserved environments on Earth, such as those living in remote regions or experiencing prolonged confinement.

Compared to Lloro V.’s 2020 systematic review [7], this study offers a more current and comprehensive assessment of the available literature. While Lloro’s review primarily focused on the effects of space and simulated microgravity on oral health, our work updates the evidence by including recent studies, identifying emerging risks, and highlighting advances in preventive strategies. Specifically, this review addresses three key questions: whether exposure to microgravity and space radiation increases the risk of dental and oral diseases/molecular changes compared to unexposed individuals; what preventive dental measures astronauts should undertake prior to mission launch; and which dental emergencies astronauts should be prepared to manage autonomously during spaceflight. By answering these focused questions, this review aims to provide relevant and up-to-date insights for researchers, space agencies, and healthcare professionals committed to preserving astronauts’ oral health during both short- and long-duration missions.

2. Materials and Methods

This systematic review was conducted by following and reporting according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses [13]. The systematic review protocol was registered on PROSPERO with the protocol identification number CRD42023472765.

2.1. Eligibility Criteria

PICO Question [14]:

Participants/Population: Human subjects involved in a space mission of any age, sex, geographic location, or specific characteristics relevant to answering the research question.

Intervention/exposure: Exposure to microgravity and space radiation.

Comparison/control: Human subjects staying on planet Earth, backup crew members, or healthy control subjects from the NASA test subject facility.

Outcomes: The effects of microgravity and space radiation on human oral health. This includes specific outcomes such as changes in saliva composition (e.g., electrolytes, protein, lysozyme, IgA, IgG), changes in salivary antimicrobial proteins and stress biomarkers (e.g., cortisol, DHEA), latent virus reactivation (e.g., EBV, CMV, VZV), and alterations in the oral microbiome. This review also sought information on how dental emergencies were managed during space missions.

Time: Patients followed for any duration of time.

We have established clear inclusion and exclusion criteria to guide the selection of relevant studies. Inclusion criteria encompass randomized clinical trials (RCTs), cohort studies, case–control studies, and cross-sectional studies, which together provide a comprehensive view of oral health in the context of space missions. These study designs allow us to investigate various aspects of oral health, from short-term impacts to long-term trends among astronauts. Conversely, exclusion criteria comprise non-human studies, qualitative studies, systematic reviews, conference abstracts, studies unrelated to oral health in the context of space exploration, and non-English language studies. By adhering to these criteria, we aim to ensure the selection of high-quality, pertinent research for our systematic review, providing a robust foundation for our analysis of oral health in space exploration.

2.2. Information Sources and Search Strategies

The electronic databases like PUBMED, Scopus, Web of Science, Cochrane Library, and OVID Medline were identified. The search for relevant articles was then conducted by following an analysis of the keywords present in the titles and abstracts of the papers. The index terms used to describe the articles were also retrieved. Additionally, the reference lists of the identified reports and articles were searched for additional sources. The search was independently conducted by S.K., M.G., M.P., and M.D.F. As the search strategies became more familiar to the working group with the evidence base, additional keywords and sources, along with potentially useful search terms, were discovered and incorporated into the search strategy. In such cases, it was of the utmost importance that the entire search strategy and results were transparent and auditable. Articles published in English were considered due to language skills restrictions.

The following keywords and search terms, connected with the Boolean operators OR and AND, were used: spaceflight*, “space flight*”, “long, duration spaceflight*”, microgravity, “partial gravity”, “low gravity”, “simulated microgravity”, aeronautics, astronaut*, astronomy, aerospace, “outer space”, “space mission”, interplanetary, “space travel*”, “space medicine”, “aviation medicine”, aviation, weightlessness, “space radiation”, NASA, ESA, “space radiation effects”, “low, shear”, “zero gravity”, “space dentistry”, oral, dental, dentistry, tooth, teeth, “dental health”, “oral health”, maxillofacial, “head and neck”, periodontitis, “dental caries”, “alveolar bone”, jaw, jawbone, “jaw bone”, mandib*, maxilla*, “maxillary sinus*”, “facial pain”, “teeth numbness”, “oral cavity”, saliva, “salivary duct”, “salivary gland”, “oral cancer”, “headache”, rash, “nasal congestion”, “acute sinusitis”, barodontalgia, barotrauma, TMJ, TMD, temporomandibular, “myofascial pain”, “posture control”, tinnitus, “jaw dislocation”, “dental emergenc*”, occlusion, “dental occlusion”, “myofascial dysfunction”, “jaw dislocation”, headache, “tension type headache”, “antibiotic resistance”, “oral microb*”, “dental biofilm”, “dental implant”, crown, “dental prosthesis”.

2.3. Selection Process

The systematic review protocol outlined the process of source selection, covering all stages based on title and abstract examination, as well as full-text examination, with specified procedures for resolving disagreements between reviewers. Selection involved independent reviews by the working group, adhering to inclusion criteria that were pre-determined in the review protocol. Discrepancies were resolved through consensus or the decision of a critical reviewer (M.D.F.) in the working group. The protocol included a narrative description of the entire review process, complemented by a flowchart following the PRISMA statement for conducting systematic reviews, detailing the flow from search to source selection, duplicates, full-text retrieval, additional searches, data extraction, and evidence presentation [14]. The software for managing search results, EndNote version 9 x USA, was used. Full-text article details were provided, and appendices were dedicated to including and briefly mentioning excluded sources, along with stated reasons for exclusion. The suggested framework for pilot testing involved randomly selecting 25 titles/abstracts, team screening based on eligibility criteria and definitions/elaboration, a team meeting to discuss discrepancies and modify criteria, and initiating screening only when 75% (or greater) agreement was achieved [13,14].

2.4. Data Collection Process

The data collection process was conducted using a logical and descriptive summary of the results aligned with the objectives and questions of the systematic review. At the protocol stage, a draft charting in Excel was developed and piloted to record key source information, including author, reference, and results or findings relevant to the review questions. This was refined at the review stage, and the Excel table format was updated accordingly. The working group chose to chart key information such as author(s), year of publication, origin/country, aims/purpose, population, sample size, methodology, intervention type, comparator details, outcomes, and key findings related to the systematic review questions. A template data extraction instrument for source details, characteristics, and result extraction was provided as a table of this review. To facilitate reference and tracking, the work group was advised to keep meticulous records for source identification. Charting the results became an iterative process with the charting table continually updated as reviewers became familiar with the source results. It was recommended that the review team pilot the extraction form on two or three sources to ensure all relevant results were extracted. This pilot step involved at least two members (S.K. and M.D.F.) of the review team and was in line with the approach favored by other authors on the conduct of systematic reviews [13,14].

2.5. Data Items

The primary data extracted from the primary studies contained various elements, including publication details, participant information, study category (topic), study type (clinical, experimental, or review), and details about intervention and comparison groups if applicable. Specifically, clinical and experimental studies were further classified based on the environment in which they were conducted, distinguishing between outer space (involving short- and long-term space missions and space stations). Data from human studies that included information such as demographics, enrollment numbers, dropout rates, and details on how dental emergencies were managed during space missions were extracted. Lastly, the review sought information on study outcomes, including primary and secondary outcomes, measurement methods, and timing.

2.6. Risk of Bias Assessment

Methodological quality of RCTs was assessed using the Cochrane Collaboration’s tool for assessing risk of bias. The ROBINS-I tool was employed to assess the risk of bias in cohort, case–control, and cross-sectional observational studies. This evaluation was independently conducted by two authors, and disagreements were resolved after consultation with the other authors. Where applicable, studies were evaluated for concealment of allocation, adequate blinding, reporting of outcome data, accounting for loss to follow-up in the analysis, reporter bias, selection bias, and other factors contributing to an increased risk of bias. Studies were then stratified based on risk of bias (low risk, high risk, indeterminate). The review was performed by evaluating the risk of bias through a qualitative analysis of the clinical studies via the National Heart, Lung, and Blood Institute (NHLBI) Quality Assessment of Controlled Intervention Studies. This approach allowed a comprehensive and systematic assessment of quality and potential bias within the included studies to establish the reliability and credibility of the results. The AMSTAR 2 checklist was used to improve the quality of reporting in the systematic review.

2.7. Effect Measures

The results from individual studies were synthesized to draw broader conclusions about the impact of space conditions on oral health. This included identifying patterns, commonalities, and discrepancies across different studies. If possible, quantitative synthesis was planned using meta-analysis techniques to calculate pooled effect sizes. However, due to potential heterogeneity in study designs and outcome measures, a narrative synthesis approach was also employed to encapsulate the findings effectively.

2.8. Synthesis Methods

Data from separate trials or studies may be combined if feasible for meta-analysis if interventions, outcome measures, and demographics are sufficiently similar. If there is no significant heterogeneity, a fixed-effect model will be applied to pool data. If significant heterogeneity exists, a random-effects model will be applied. Data with a high degree of heterogeneity will not be pooled. If appropriate and there are sufficient data for subgroup analysis, then subgroup analyses were planned based on the duration of flight, i.e., <6 months and >6 months [13,14].

2.9. Certainty Assessment

The approach to certainty assessment consisted of the selection of appropriate frameworks and tools tailored to the review’s requirements. GRADE, a framework for evaluating evidence quality and recommendation strength, was primarily employed. This framework considers various factors such as risk of bias, inconsistency, indirectness, imprecision, and publication bias. Additionally, the Cochrane Risk of Bias Tool was utilized for assessing bias in randomized trials, alongside other specialized tools such as the ROBINS-I tool and the Newcastle–Ottawa scale for non-randomized studies and observational studies, respectively.

Study quality assessment involved a thorough examination of each study’s design and methodology, focusing on potential biases like selection, performance, detection, attrition, and reporting biases. Consistency among study results was filtered, with significant heterogeneity impacting the certainty of the evidence. Directness was also evaluated, making sure the evidence directly addressed the review’s question in terms of population, intervention, comparators, and outcomes.

Additional factors considered included the precision of the evidence, assessed through confidence intervals and sample sizes, and the potential for publication bias. The findings for each outcome were summarized, indicating the level of certainty of the evidence [13,14].

3. Results

A total of 13 studies were identified, and 7 were ultimately included for qualitative synthesis [15,16,17,18,19,20,21]; the experiments were conducted in simulated labs. A total of N = 7 studies were included for qualitative synthesis of clinical studies. Quantitative data were available from only two studies [15,18], and due to differing outcomes, it was not feasible to conduct a meta-analysis. Figure 1 presents the PRISMA flowchart outlining the identification, screening, eligibility, and inclusion steps.

Lee R. Brown (1977) studied the chemical composition of saliva in astronauts, marking an early exploration into physiological changes during space travel, as published in the Journal of Dental Research [15]. Agha (2020) focused on the impact of long-duration spaceflight on salivary antimicrobial proteins and stress biomarkers, a study that was featured in the Journal of Applied Physiology [16]. Mehta (2014, 2017, 2013) contributed a series of studies across different years, addressing various aspects of microbiology and immunology in space, with publications in Brain, Behavior, and Immunity, NPJ Microgravity, and Cytokine [17,18,19]. Voorhies (2019) provided a significant contribution, offering fresh insights and perspectives in the field [20]. Buchheim (2019) further enriched space research with their study, enhancing our understanding of the unique challenges and physiological changes associated with space travel [21].

The studies in the provided data predominantly originate from two countries: the United States (USA) and Germany. The literature included in this review related to space was of different types of study designs. These include prospective observational cohort studies, longitudinal observational studies, and observational cohort studies.

The studies used a diverse range of samples, including “Whole Unstimulated Saliva,” providing insights into oral health and systemic physiological changes. The collection of “Saliva, Blood, and Urine” allowed researchers to analyze various biomarkers and physiological responses to space conditions. Additionally, studies using “Blood and Saliva Samples” enabled the examination of both systemic and localized responses in the body.

The total number of crew members and participants included in the studies was N = 110. Lee R. Brown (1977) conducted research involving nine prime and nine backup crew members selected by the Johnson Space Center for the three Skylab missions [15]. Agha (2020)’s study included ISS crewmembers (seven men, one woman, four rookies, and four veterans) and six healthy ground-based control subjects (five men, one woman) [16]. Mehta (2014) focused on 17 ISS astronauts in his study [17]. Mehta (2017)’s research involved 23 ISS astronauts [18]. Mehta (2013) conducted a study with 27 ISS astronauts [19]. Voorhies (2019) included nine astronauts who spent six to twelve months in the ISS in his research [20]. Buchheim (2019)’s study included 12 participants [21].

Lee R. Brown (1977) investigated a “Space mission” as the intervention, with “backup crew members” serving as the comparator group [15]. Agha (2020) studied a “six-month mission to the International Space Station (ISS)” as the intervention, with “six healthy ground-based control subjects (five men, one woman)” serving as comparators [16]. Mehta (2014) examined a “short Space mission” compared against “healthy subjects” [17]. Mehta (2017) studied a “long Space mission,” with “20 healthy subjects” as the comparator group [18]. Mehta (2013) evaluated a “short Space mission,” with the comparator group consisting of “10 healthy control subjects from the NASA Test Subject Facility” [19]. In Buchheim (2019), specific details for both the intervention and comparator were not mentioned [21].

Lee R. Brown (1977) included saliva levels of electrolytes, total protein, albumin, IgA, IgG, and lysozyme [15]. Agha (2020) assessed changes in salivary antimicrobial proteins and stress biomarkers during a 6-month mission to the International Space Station, including salivary cortisol, DHEA, C-reactive protein (CRP), salivary secretory IgA (sIgA), α-amylase activity, LL-37, HNP 1–3, lactoferrin, and lysozyme [16]. Mehta (2014) investigated latent virus reactivation and diurnal salivary cortisol and dehydroepiandrosterone [17]. Mehta (2017) focused on viral reactivation, EBV DNA levels in peripheral blood mononuclear cells, salivary cortisol, DHEA, antiviral antibodies, and plasma cortisol [18]. Mehta (2013) included latent virus reactivation, plasma cytokines, and the ratio of IFN c and IL4 in shedders and non-shedders before and after spaceflight [19]. Voorhies (2019) focused on astronauts’ microbiome, collected at various times before, during, and after spaceflight [20]. Buchheim (2019) measured saliva levels of cortisol, blood cell concentration, and cytokine [21].

The studies were conducted over varying durations. Lee R. Brown (1977) had a study duration of 77 days [15]; Agha (2020) conducted an experiment lasting 246 days [16]; Mehta (2014) spanned 210 days [17]; Mehta (2017) conducted a longer study lasting 390 days [18]; Mehta (2013) lasted 254 days [19]; Voorhies (2019) conducted one of the longest studies, extending over 780 days [20]; and Buchheim (2019) had a duration of 236 days [21].

Lee R. Brown (1977) utilized “Atomic absorption analysis” [15]. Agha (2020) employed the “ELISA Kit” technique, commonly used for detecting and quantifying substances such as proteins, hormones, and antibodies [16]. Mehta (2014) (2013), Voorhies (2019), and Buchheim (2019) used “PCR” (polymerase chain reaction), a method widely employed in molecular biology to amplify DNA sequences [17,20,21]. Mehta (2017) utilized a “Nonorganic extraction method” with QIAamp Viral RNA Kits for extracting viral genomic DNA, indicating a focus on genetic analysis [18]. The absolute effects observed in viral shedding (EBV, VZV, and CMV) during and after spaceflight, based on data from Mehta et al. (2017) [18], are presented in

Table 1. Absolute and relative effects calculated from the available eligible studies.

Outcomes	Anticipated Absolute Effects * (95% CI)	Relative Effect (95% CI)	No. of Participants in Studies
VZV copies/ng salivary DNA	Absolute effects during flight (Mid): 0.816, after flight (R + 0): 0.1300	Undefined	23
EBV copies/ng salivary DNA	Absolute effects during flight (Mid): 0.1020, after flight (R + 0): 0.770	Undefined	23
CMV copies/ng urinary DNA	Absolute effects during flight: 0.467, after flight (R + 0): 0.56, after flight (R + 30): 0.450	Undefined	23

* The risk in the intervention group (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). The data were available from the Mehta 2017 study.

.

3.1. Risk of Bias

Most studies, including Brown 1977, Agha 2020, Mehta 2013, 2014, and 2017, and Buchheim 2019, were overall rated as having a moderate risk of bias [15,16,17,18,19,21]. The Voorhies 2019 study scored a low risk of bias [20]. The only domain with a moderate risk of bias for Brown 1977, Agha 2020, Mehta 2013, 2014, and 2017, and Buchheim 2019 [15,16,17,18,19,21] was the selection of participants domain (Figure 2).

3.2. Certainty of Evidence

The certainty of evidence for considered outcomes was low (

Table 2. Risk Effects in Space.

Study	Location of Experiment	Participants	Intervention	Outcomes	Duration of Experiment (Days)	Key Results	Certainty of Evidence
Lee R. Brown 1977 [15]	International Space Station (ISS)	9 prime, 9 backup crew members	Space mission, Skylab missions	Saliva composition (electrolytes, protein, IgA, IgG, lysozyme)	77	Increased saliva flow rate, age and gender influencing saliva flow rates, significant lysozyme diminution, changes in various saliva components	Low
Agha 2020 [16]	International Space Station (ISS)	ISS crew members, ground-based control subjects	6-month mission to ISS	Changes in salivary antimicrobial proteins, stress biomarkers	246	Salivary shedding of viruses, lower cortisol levels in the test group, increased salivary sIgA concentration and secretion, elevated lysozyme levels	Low
Mehta 2014 [17]	International Space Station (ISS)	17 ISS astronauts	Short Space Mission	Latent virus reactivation, salivary cortisol, DHEA	210	No significant change in salivary cortisol levels, presence of EBV viral shedding, increased DHEA during flight	Low
Mehta 2017 [18]	International Space Station (ISS)	23 ISS astronauts	Long Space Mission	Viral reactivation, EBV DNA levels, salivary cortisol, DHEA	390	Viral reactivation during spaceflight, changes in cortisol levels during the mission	Low
Mehta 2013 [19]	International Space Station (ISS)	27 ISS Astronauts, 10 healthy control subjects	Short Space Mission	Latent virus reactivation, plasma cytokines	254	Elevated levels of cytokines in virus-shedding astronauts and a link between virus reactivation and Th2 cytokine balance	Low
Voorhies 2019 [20]	International Space Station (ISS)	9 astronauts on long-duration ISS missions	Long Space Mission	Impact on astronaut microbiome	780	Changes in spatial microbial patterns, alterations in microbial composition, effects on alpha and beta diversity, interaction with immune dysregulation	Low
Buchheim 2019 [21]	Gagarin Cosmonaut Training Centre (GCTC) and ISS	12 Russian cosmonauts	Long-Duration Spaceflight	Saliva cortisol levels, blood cell concentration, cytokine levels	Not specified	Subjective stress levels, circadian differences in cortisol levels, changes in endocannabinoid levels, alterations in peripheral blood leukocyte phenotypes, activation of polymorphonuclear leukocytes, amplified TNF response to fungal antigen, regulatory T-cell subsets, and cytokine profiles	Low

).

4. Discussion

4.1. Summary of Main Results

Thirteen studies met the inclusion criteria; however, six were excluded due to article retractions and the use of simulated laboratory settings. Therefore, seven clinical studies were included in the qualitative synthesis. Although two studies [15,18] provided quantitative data, a meta-analysis was not feasible due to differences in outcomes and study design.

The literature offered a range of findings relevant to the research questions and objectives of this review. Various studies investigated how space missions affect astronauts’ oral health, including changes in salivary composition during the Skylab missions [15] and increased levels of salivary antimicrobial proteins and stress biomarkers during a six-month mission aboard the International Space Station (ISS) [16]. Latent virus reactivation was also documented: several studies reported the reactivation of Epstein–Barr virus (EBV), cytomegalovirus (CMV), and varicella zoster virus (VZV) during both Space Shuttle and ISS missions, correlating with elevated plasma cytokines [17]. The astronaut oral microbiome was examined in relation to long-duration space missions at the ISS [18,19], and stress-induced immunological alterations resembling chronic, low-grade inflammation that increases with age were observed in cosmonauts following extended spaceflight [21].

Brown et al. (1977) [15] found that saliva flow rates increased in the later phases of space missions, possibly due to stress, adaptation to microgravity, or mission-specific factors. Age and sex were identified as covariates influencing these outcomes. Salivary protein levels, particularly albumin and IgG, remained stable throughout, suggesting a degree of resilience to environmental changes. Lysozyme, a small constituent of total salivary protein, declined significantly during preflight and continued to decrease throughout the mission, especially on 28- to 84-day flights. This decline may have immunological implications. In contrast, salivary IgA levels increased prior to flight and peaked post-mission, with interindividual variability observed during the 84-day mission, indicating diverse immune responses to space exposure.

Agha et al. (2020) [16] reported that 50% of ISS crew members shed viruses in saliva or urine during spaceflight, with 38% testing positive for herpesviruses. Lower levels of cortisol and DHEA were observed, alongside elevated salivary secretory IgA (sIgA) and lysozyme, suggesting unique physiological adaptations to the space environment. Other studies confirmed viral shedding during space missions, including EBV, CMV, and VZV, with the latter two more prevalent during and after spaceflight. While salivary cortisol levels changed mid-mission, anti-EBV and anti-CMV antibody titers remained stable [17,18].

Research by Mehta and colleagues provided key insights into latent virus reactivation. Mehta et al. (2013) linked increased cytokine levels with viral shedding and a shift toward Th2-type immune responses during short-duration flights. Mehta (2014) found no correlation between diurnal changes in salivary cortisol/DHEA and viral reactivation. Mehta (2017) confirmed significant viral shedding (EBV, CMV, VZV) among ISS astronauts, accompanied by changes in salivary cortisol levels [17,18,19].

Voorhies et al. (2019) [20] conducted a longitudinal study on nine astronauts during long-duration ISS missions (6–12 months). Whole unstimulated saliva samples were collected at multiple time points and analyzed via PCR. Findings showed increased gastrointestinal microbiome diversity and decreased nasal microbiota diversity, both returning to baseline post-mission. However, notable post-flight shifts in microbial taxa (e.g., Firmicutes and Proteobacteria) persisted, and changes correlated with immune dysregulation and psychological stress.

Finally, Buchheim et al. (2019) [21] assessed twelve Russian cosmonauts before, during, and after ISS missions. Using stress questionnaires, PCR analysis of saliva and blood, and immune profiling, the study revealed modest self-reported stress but significant biological changes. These included elevated leukocyte counts, activated polymorphonuclear cells, enhanced TNF-α responses to fungal antigens, and altered regulatory T-cell and cytokine profiles. These findings highlight the complex physiological and immunological impact of long-term spaceflight.

4.1.1. Biophysical Mechanisms

Microgravity modifies cytoskeletal dynamics and fluid mechanics, affecting mechanotransduction, nuclear architecture, and transcriptional regulation. Radiation induces DNA double-strand breaks and oxidative damage to proteins and lipids, impairing immune and salivary gland function. Combined, these factors explain the observed biomarker shifts, immune dysregulation, and oral-systemic changes that could have impact on both hard and soft tissue changes. Some of these include the following;

Salivary Protein Alterations

Across the included studies, astronauts demonstrated variations in salivary protein composition, particularly in lysozyme and immunoglobulin A (IgA). These proteins are central to oral antimicrobial defense, and their altered concentrations may predispose astronauts to microbial overgrowth and reduced mucosal immunity. From a biophysical point of view, microgravity modifies fluid shear forces and diffusion gradients, potentially disrupting protein folding and stability. Such changes may impair enzymatic activity (e.g., α-amylase) and alter the functional capacity of salivary proteins, thereby weakening the first line of oral defense during missions.

4.1.2. Immune Dysregulation

Evidence consistently shows spaceflight-associated immune alterations, including skewing toward a T-helper 2 (Th2) cytokine profile and reduced antiviral surveillance. These findings may be explained by microgravity-induced cytoskeletal remodeling in immune cells, which disrupts mechanotransduction pathways. Altered actin dynamics can impair receptor clustering and intracellular signaling cascades (NF-κB, MAPK), thereby reducing T-cell activation and proliferation. The consequence is immune suppression, particularly in antiviral responses, which aligns with increased viral shedding reported in astronauts.

4.1.3. Viral Reactivation

Latent viruses, including Epstein–Barr virus (EBV), cytomegalovirus (CMV), and varicella zoster virus (VZV), have been shown to reactivate during long-duration missions. Viral shedding in saliva and blood reflects compromised immune surveillance under conditions of stress and immune imbalance. Biophysically, spaceflight-induced gene expression changes, partly mediated by altered chromatin architecture and oxidative stress, may downregulate antiviral genes and increase susceptibility to reactivation. This phenomenon represents a key intersection of environmental stress, gene regulation, and immune dysregulation in space.

4.1.4. Microbiome Dysbiosis

Studies of astronaut saliva and gastrointestinal samples revealed changes in microbial composition, with increased Firmicutes and decreased Proteobacteria. Such shifts might be attributed to altered mucosal shear stress in microgravity, which modifies microbial niche environments, and to radiation-induced DNA damage in commensal microbiota, selecting for more stress-resistant species. This dysbiosis can further exacerbate immune imbalance and inflammation, contributing to oral and systemic health risks during prolonged missions [22,23,24].

4.1.5. Stress Biomarkers

Cortisol and dehydroepiandrosterone (DHEA) levels fluctuated during missions, reflecting altered stress-immune interactions. These endocrine markers are critical regulators of inflammation, viral reactivation, and mucosal immunity. The observed fluctuations likely result from a combination of mechanical stress (disrupted mechanotransduction in neuroendocrine pathways) and oxidative stress (radiation-induced damage to endocrine tissues). Together, these changes highlight the endocrine-immune crosstalk disrupted in spaceflight, providing a physiological link between psychological stress, immune dysregulation, and oral health outcomes.

4.2. Different Comparative Studies

The reviewed literature consisted of a variety of objectives, outcomes, methodologies, and conclusions. One study investigated salivary electrolyte levels, total protein, albumin, immunoglobulins (IgA and IgG), and lysozyme [15]. Another evaluated changes in salivary antimicrobial proteins, stress biomarkers, cortisol, dehydroepiandrosterone (DHEA), C-reactive protein (CRP), secretory IgA (sIgA), alpha-amylase, LL-37, HNP 1–3, lactoferrin, and lysozyme, along with psychological assessments using the Profile of Mood States (POMS) and the Pittsburgh Sleep Quality Index (PSQI) [16]. Other investigations focused on latent virus reactivation and variations in diurnal salivary cortisol and DHEA [17], viral reactivation with Epstein–Barr virus (EBV) DNA levels, salivary and plasma cortisol, DHEA, and antiviral antibody titers [18], as well as latent virus reactivation in relation to plasma cytokine levels and the IFN-γ/IL-4 ratio in virus shedders versus non-shedders [19]. Additional work included longitudinal microbiome analyses at multiple time points during long-duration space missions [20], and the measurement of salivary cortisol, blood cell concentrations, cytokine profiles, and responses to standardized stress questionnaires [21].

4.3. Risk Effects on Outcomes in Space

The effects of space missions on microbiological and immunological parameters are detailed in Table 2, highlighting key alterations observed in astronauts during and after spaceflight.

4.4. Overall Completeness and Applicability of Evidence

The included studies include a wide range of outcomes, including salivary composition, immune system responses, stress biomarkers, and microbiome alterations. This diversity strengthens the evidence, offering an overview of the physiological changes associated with space missions. The studies feature participants with diverse demographics varying in age, gender, and spaceflight experience, enhancing the representativeness and applicability of the findings. These aspects are synthesized in Table 2, which provides an overview of the experimental settings, key outcomes, and the certainty of evidence across the included studies.

The mission durations examined range from short-term flights (77 days) to long-duration missions (up to 780 days), allowing for a comparative assessment of acute versus chronic effects. The use of varied analytical methodologies, including atomic absorption spectroscopy, ELISA, PCR, and advanced microbiome profiling, adds methodological richness and improves the robustness of the evidence.

Temporal dimensions are also well integrated into the study designs, with data collection points occurring before, during, and after spaceflight. This temporal coverage enables detailed observation of baseline conditions, in-flight physiological changes, and post-flight recovery, contributing to a better understanding of spaceflight’s impact on human biology.

4.5. Overall Applicability

The included studies were conducted in real spaceflight conditions, primarily aboard the International Space Station (ISS), which enhances the relevance and applicability of the evidence to the actual environment experienced by astronauts. Several studies incorporated ground-based control groups, allowing for meaningful comparisons between spaceflight, induced changes, and Earth-based physiological baselines. This comparative approach broadens the applicability of findings beyond the space context, offering insights into general human physiological responses.

Clear inclusion and exclusion criteria were consistently reported, ensuring that study participants met defined health and fitness standards. This methodological aspect increases the relevance of the findings to the astronaut population, which undergoes similar selection protocols. Moreover, the inclusion of participants from different countries, such as the United States and Russia, adds international diversity and strengthens the evidence for astronauts of various national backgrounds.

Finally, the studies examined both short- and long-duration missions, reflecting the variable nature of current and future space travel. This range enhances the applicability of the results to different mission profiles, from brief orbital flights to extended stays aboard the ISS or future deep space expeditions.

4.6. Quality of Evidence

Although several studies have explored microbiological and immunological aspects of spaceflight, the overall quality of evidence remains low. Further research is required to investigate specific physiological processes—such as hemostatic mechanisms—in the space environment, which remain largely unexamined.

4.7. Agreements and Disagreements with Other Studies or Reviews

In comparing the findings of our systematic review with those of Lloro et al. (2020) [7], several points of agreement and divergence emerge.

4.7.1. Agreements

Both reviews highlight the significant impact of space missions on oral health, particularly in the context of long-duration spaceflights. Consistently, both studies identify microgravity as a key factor influencing oral health by inducing a range of physiological changes. There is agreement on the occurrence of specific oral conditions during space missions, including dental pain, dental caries, and alterations in salivary composition. Furthermore, both reviews highlight the role of saliva analysis as a valuable diagnostic tool, emphasizing how spaceflight, related changes in salivary components can offer insights into the overall oral and systemic health of astronauts.

4.7.2. Disagreements

Our review specifically highlights salivary viral shedding, particularly Epstein–Barr virus (EBV), and the association between viral reactivation and a Th2-skewed cytokine balance as critical findings. Our results provide a more in-depth analysis of the immunological dimension, including viral shedding and cytokine balance, which was not the primary focus of Lloro et al. (2020). While Lloro et al. provided a broader categorization of oral health events occurring during space missions, including incapacitating dental pain, dental caries, and alterations in microbial flora, our review places greater emphasis on immunological dynamics and the mechanisms of viral activation. In particular, we explored deeper into the interplay between cytokine release patterns and oral health outcomes in the space environment, a connection that is only marginally addressed in Lloro et al.’s work.

Although both reviews coincide on the overarching impact of space missions on oral health and recognize the significant role of microgravity in inducing physiological changes, they differ in their focal points. Lloro et al. presented anoverview of oral health conditions linked to spaceflight, whereas our review contributes a more immunologically focused analysis, emphasizing virus reactivation and immune response modulation. These complementary perspectives help to build a more complete understanding of the multifactorial nature of oral health challenges in space exploration.

4.8. Operational and Clinical Implications

Our findings have direct implications for mission planning. Preventive dental care should be prioritized prior to launch, including comprehensive examinations, removal of carious lesions, stabilization of restorations, use of type of local anaesthetics, extraction of compromised teeth, and application of preventive measures such as fluoride varnish. Astronauts should be trained in emergency dental procedures [25], supported by onboard dental emergency kits containing temporary filling materials, anaesthetic options, and extraction tools. Importantly, saliva-based diagnostics could be integrated as a non-invasive monitoring tool for immune dysregulation and viral reactivation during space missions, complementing existing health surveillance systems.

4.9. Limitations

Several limitations must be acknowledged. Most included studies are observational with small, heterogeneous samples. Confounding factors such as diet, oral hygiene, baseline microbiome, and psychological stress were not consistently controlled. Importantly, NASA missions lack in-flight 1 g controls, limiting our ability to distinguish between microgravity and radiation effects. Findings are therefore most applicable to LEO missions and may not generalize to deep-space environments such as the Moon or Mars, where radiation exposure and mission durations will differ substantially. The certainty of evidence remains low to moderate.

5. Conclusions

There are variations in the chemical composition of saliva and in saliva flow rates. In terms of virus activation, there was salivary shedding of viruses, the presence of EBV viral shedding, elevated levels of cytokines in virus-shedding astronauts, a link between virus reactivation and Th2 cytokine balance, and other immunological changes like cytokine release. This suggests that long-duration space missions are associated with alterations in salivary biomarkers, viral reactivation, and oral microbiome dynamics which could impact on outcomes related to dental and its supporting structures. These changes appear linked to stress and immune dysregulation, though causality cannot be established. The evidence is limited by small heterogeneous samples and the absence of controlled studies, making conclusions indicative rather than definitive. Preventive dental measures and emergency preparedness remain essential for astronaut safety.