Estimating safe human exposure levels for lunar dust using benchmark dose modeling of data from inhalation studies in rats

Contemporary review of dermatologic conditions in space flight and future implications for long-duration exploration missions

BACKGROUND

Future planned exploration missions to outer space will almost surely require the longest periods of continuous space exposure by the human body yet. As the most external organ, the skin seems the most vulnerable to injury. Therefore, discussion of the dermatological implications of such extended-duration missions is critical.

OBJECTIVES

In order to help future missions understand the risks of spaceflight on the human skin, this review aims to consolidate data from the current literature pertaining to the space environment and its physiologic effects on skin, describe all reported dermatologic manifestations in spaceflight, and extrapolate this information to longer-duration mission.

METHODS AND MATERIALS

The authors searched PubMed and Google Scholar using keywords and Mesh terms. The publications that were found to be relevant to the objectives were included and described.

RESULTS

The space environment causes changes in the skin at the cellular level by thinning the epidermis, altering wound healing, and dysregulating the immune system. Clinically, dermatological conditions represented the most common medical issues occurring in spaceflight. We predict that as exploration missions increase in duration, astronauts will experience further physiological changes and an increased rate and severity of adverse events.

CONCLUSION

Maximizing astronaut safety requires a continued knowledge of the human body's response to space, as well as consideration and prediction of future events. Dermatologic effects of space missions comprise the majority of health-related issues arising on missions to outer space, and these issues are likely to become more prominent with increasing time spent in space. Improvements in hygiene may mitigate some of these conditions.

Overview of lunar dust toxicity risk

Lunar dust (LD), the component of lunar regolith with particle sizes less than 20 μm, covers the surface of the Moon. Due to its fineness, jagged edges, and electrostatic charge, LD adheres to and coats almost any surface it contacts. As a result, LD poses known risks to the proper functioning of electronic and mechanical equipment on the lunar surface. However, its mechanical irritancy and chemical reactivity may also pose serious health risks to humans by a number of mechanisms. While Apollo astronauts reported mild short-lived respiratory symptoms, the spectrum of health effects associated with high-dose acute exposure or chronic low-dose exposure are not yet well-understood. This paper explores known and potential human risks of exposure to LD which are thought to be important in planning upcoming lunar missions and planetary surface work.

Comparative pulmonary toxicities of lunar dusts and terrestrial dusts (TiO2 & SiO2) in rats and an assessment of the impact of particle-generated oxidants on the dusts' toxicities

Humans will set foot on the Moon again soon. The lunar dust (LD) is potentially reactive and could pose an inhalation hazard to lunar explorers. We elucidated LD toxicity and investigated the toxicological impact of particle surface reactivity (SR) using three LDs, quartz, and TiO₂. We first isolated the respirable-size-fraction of an Apollo-14 regolith and ground two coarser samples to produce fine LDs with increased SR. SR measurements of these five respirable-sized dusts, determined by their in-vitro ability to generate hydroxyl radicals (•OH), showed that ground LDs > unground LD ≥ TiO₂ ≥ quartz. Rats were each intratracheally instilled with 0, 1, 2.5, or 7.5 mg of a test dust. Toxicity biomarkers and histopathology were assessed up to 13 weeks after the bolus instillation. All dusts caused dose-dependent-increases in pulmonary lesions and toxicity biomarkers. The three LDs, which possessed mineral compositions/properties similar to Arizona volcanic ash, were moderately toxic. Despite a 14-fold •OH difference among these three LDs, their toxicities were indistinguishable. Quartz produced the lowest •OH amount but showed the greatest toxicity. Our results showed no correlation between the toxicity of mineral dusts and their ability to generate free radicals. We also showed that the amounts of oxidants per neutrophil increased with doses, time and the cytotoxicity of the dusts in the lung, which supports our postulation that dust-elicited neutrophilia is the major persistent source of oxidative stress. These results and the discussion of the crucial roles of the short-lived, continuously replenished neutrophils in dust-induced pathogenesis are presented.

Effects of lunar dust simulant on cardiac function and fibrosis in rats

The objective of this study was to investigate the effects of lunar dust simulant (LDS) on cardiac function and fibrosis. In an in vivo experiment, after 3 weeks of exposure, electrocardiography (ECG) and histopathological and immunohistochemical analyses of the cardiac tissue were performed. Systemic inflammatory markers and genes and proteins associated with cardiac tissue fibrosis were examined. In an in vitro experiment, fibrosis-related factors were detected in H9c2 cells by western blot and the mechanism of myocardial fibrosis by LDS exposure was explored. LDS exposure significantly altered heart rate indicators, implying altered cardiac and autonomic system functions. LDS dose-dependently increased the type and number of ECG abnormalities, and increased serum inflammatory factors. In addition, pathological changes in the myocardial tissue were observed through hematoxylin and eosin, Masson, and immunohistochemical staining; the expression of genes and proteins related to fibrosis in the myocardial tissue was also altered. These findings indicate that LDS exposure causes systemic inflammatory lesions that affect autonomic function, leading to inflammatory myocardial fibrosis. And its mechanisms involve the mediation of the nuclear factor-E2-related factor (Nrf2)/nicotinamide adenine dinucleotide phosphate oxidase 4 (NOX4) redox balance.

Express assessment of neurotoxicity of particles of planetary and interstellar dust

Establishment of high-quality, consistent on-board assessment of the neurotoxicity of planetary, and interstellar dust particles will be required to predict their potential threat to human health during long-term space missions. This Perspective article proposes an approach for the rapid assessment of potential neurotoxicity of micro-sized and nano-sized dust particles based on experimental results with other neurotoxic particles. Capacity of particles to affect membrane potential, integrity of nerve terminals, and consequently key synaptic transmission characteristics can be assessed using a planar lipid bilayer technique by monitoring artificial membrane conductivity in the presence of particles. Preliminary neurotoxicity data of nanoparticles, including lunar and Martian dust simulants, obtained using a planar lipid bilayer technique, is compared with that acquired using more-established methodological approaches. Under space flight conditions, neurotoxicity assessments of particulate matter could be rapidly and reproducibly performed using a planar lipid bilayer technique, which does not require biological material.

Increase in relative deposition of fine particles in the rat lung periphery in the absence of gravity

While it is well recognized that pulmonary deposition of inhaled particles is lowered in microgravity (μG) compared with gravity on the ground (1G), the absence of sedimentation causes fine particles to penetrate deeper in the lung in μG. Using quantitative magnetic resonance imaging (MRI), we determined the effect of gravity on peripheral deposition (DEPperipheral) of fine particles. Aerosolized 0.95-μm-diameter ferric oxide particles were delivered to spontaneously breathing rats placed in plethysmographic chambers both in μG aboard the NASA Microgravity Research Aircraft and at 1G. Following exposure, lungs were perfusion fixed, fluid filled, and imaged in a 3T MR scanner. The MR signal decay rate, R2*, was measured in each voxel of the left lung from which particle deposition (DEP) was determined based on a calibration curve. Regional deposition was assessed by comparing DEP between the outer (DEPperipheral) and inner (DEPcentral) areas on each slice, and expressed as the central-to-peripheral ratio. Total lung deposition tended to be lower in μG compared with 1G (1.01 ± 0.52 vs. 1.43 ± 0.52 μg/ml, P = 0.1). In μG, DEPperipheral was larger than DEPcentral (P < 0.03), while, in 1G, DEPperipheral was not significantly different from DEPcentral. Finally, central-to-peripheral ratio was significantly less in μG than in 1G (P ≤ 0.05). These data show a larger fraction of fine particles depositing peripherally in μG than in 1G, likely beyond the large- and medium-sized airways. Although not measured, the difference in the spatial distribution of deposited particles between μG and 1G could also affect particle retention rates, with an increase in retention for particles deposited more peripherally.