Olivine Dissolution in Simulated Lung and Gastric Fluid as an Analog to the Behavior of Lunar Particulate Matter Inside the Human Respiratory and Gastrointestinal Systems

Our plain-English summary of Olivine Dissolution in Simulated Lung and Gastric Fluid as an Analog to the Behavior of Lunar Particulate Matter Inside the Human Respiratory and Gastrointestinal Systems, by D.A. Hendrix, J.A. Hurowitz, T.D. Glotch, et al (GeoHealth, 2021).

The study

• Authors: D.A. Hendrix, J.A. Hurowitz, T.D. Glotch, M.A.A. Schoonen
• Journal: GeoHealth
• Published: 2021;5(11):e2021GH000491
• Cited by: 6 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

With the Artemis III mission scheduled to land humans on the Moon in 2025, work must be done to understand the hazards lunar dust inhalation would pose to humans. In this study, San Carlos olivine was used as an analog of lunar olivine, a common component of lunar dust. Olivine was dissolved in a flow-through apparatus in both simulated lung fluid and 0.1 M HCl (simulated gastric fluid) over a period of approximately 2 weeks at physiological temperature, 37°C. Effluent samples were collected periodically and analyzed for pH, iron, silicon, and magnesium ion concentrations. The dissolution rate data derived from our measurements allow us to estimate that an inhaled 1.0 μm diameter olivine particle would take approximately 24 years to dissolve in the human lungs and approximately 3 weeks to dissolve in gastric fluid. Results revealed that inhaled olivine particles may generate the toxic chemical, hydroxyl radical, for up to 5-6 days in lung fluid. Olivine dissolved in 0.1 M HCl for 2 weeks transformed to an amorphous silica-rich solid plus the ferric iron oxy-hydroxide ferrihydrite. Olivine dissolved in simulated lung fluid shows no detectable change in composition or crystallinity. Equilibrium thermodynamic models indicate that olivine in the human lungs can precipitate secondary minerals with fibrous crystal structures that have the potential to induce detrimental health effects similar to asbestos exposure. Our work indicates that inhaled lunar dust containing olivine can settle in the human lungs for years and could induce long-term potential health effects like that of silicosis.

© 2021 The Authors. GeoHealth published by Wiley Periodicals LLC on behalf of American Geophysical Union.

In plain English

In laboratory tests, the authors measured how fast olivine, a common lunar dust mineral, dissolves in simulated body fluids. No people or animals were studied. Simulated lung fluid (six runs) or 0.1 M hydrochloric acid as simulated gastric fluid (five runs) flowed through ground olivine at 37°C for about two weeks. Olivine dissolved about 400 times more slowly in lung fluid. The authors estimated that a 1 micrometer particle would take about 24 years to dissolve in the lung, versus less than one month in acid. Olivine in acid turned into silica-rich material plus ferrihydrite; olivine in lung fluid looked unchanged in microscope, elemental and infrared tests, though its surface area rose slightly. Olivine produced hydroxyl radicals, a reactive chemical, whose levels dropped sharply after about a day. Computer modeling indicated lung-fluid outputs were saturated with minerals including talc, sepiolite and, in the authors' table, chrysotile. In lung-fluid samples, one X-ray signal suggested a possible magnesium sheet mineral, but none was confirmed. The authors concluded that inhaled lunar olivine could persist in the lungs for years, raising long-term health concerns.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory (in vitro) dissolution experiment using a flow-through reactor, with separate hydroxyl radical incubation tests and geochemical computer modeling (PHREEQC). No human or animal subjects.
• Who was studied: Ground olivine of composition Fo91, used as an analog of lunar olivine. The methods say it came from a mineral collection at Mount Holyoke College. There were five runs in 0.1 M HCl and six runs in simulated lung fluid, with 46.1 to 91.9 mg of olivine per run (Table 2). Median grain size was about 7.55 plus or minus 0.72 micrometers.
• Exposure: Olivine (not an asbestos fiber) exposed at 37°C to two fluids. The first was simulated lung fluid (modified Gamble's solution, pH 7.4). The second was 0.1 M hydrochloric acid (pH 1.3) as a stand-in for gastric fluid. The flow rate was 0.13 mL per hour for about two weeks. Separate 200 mg portions were incubated for 0 to 7 days to measure hydroxyl radicals.
• Main results:
  • Average dissolution rate based on magnesium: in 0.1 M HCl, 6.5 plus or minus 1.0 × 10 to the minus 9 mol per square meter per second. In simulated lung fluid, 1.5 plus or minus 0.20 × 10 to the minus 11 mol per square meter per second. The authors state that olivine dissolved about 400 times slower in simulated lung fluid.
  • Estimated time to dissolve fully: about 24 years for a spherical 1 micrometer olivine particle in simulated lung fluid, and less than one month in 0.1 M HCl.
  • Olivine dissolved in 0.1 M HCl left an amorphous-looking, silica-rich precipitate on the surface, with infrared features matching ferrihydrite. Its surface area rose from about 3.0 to an average of 61.6 square meters per gram.
  • Olivine dissolved in simulated lung fluid looked the same as unreacted olivine under the electron microscope, in elemental (EDX) spectra and in infrared spectra. Its surface area rose from about 3.0 to 4.56 square meters per gram, which the authors, together with the X-ray signal, took as indicating that precipitation reactions may be occurring.
  • Hydroxyl radical generation by olivine was initially about double in 0.1 M HCl compared with simulated lung fluid. In both fluids it subsided rapidly after about 24 hours. The authors interpreted the lung-fluid samples as staying reactive at low levels for 5 to 6 days.
  • In the simulated lung fluid samples, X-ray diffraction showed a broad peak at about 13 to 14 degrees. The authors said this could possibly indicate a poorly crystalline magnesium phyllosilicate (examples given: chrysotile, clinochlore, chamosite). No such mineral was confirmed.
  • Modeling showed the simulated lung fluid outputs saturated with respect to iron oxides (ferrihydrite, goethite, hematite), brucite, talc and sepiolite. Table 5 also lists chrysotile among modeled phases. The 0.1 M HCl outputs were saturated with respect to goethite, hematite and quartz.
• The authors' conclusion: The authors concluded that a 1 micrometer olivine particle could stay in the lung for about 24 years, raising concerns for astronauts. They suggested that precipitation of minerals such as chrysotile and sepiolite may have adverse health implications because of a fibrous structure shared with asbestos. They also concluded that the long-term implications are not well understood.
• Limitations the authors state:
  • The simulated lung fluid lacked the complex proteins found in real lung fluid. The authors state this may lead to underestimating how available toxic trace metals are.
  • The 0.1 M HCl lacked the dissolved sodium chloride of standard simulated gastric fluid. The authors call the gastric results a first approximation.
  • Steady state may not have been reached in simulated lung fluid runs 3 and 4, so those runs were excluded from the rate calculation.
  • Iron was below the detection limit in two simulated lung fluid runs (runs 5 and 6).
  • Only the 0- and 24-hour hydroxyl radical incubations could be reliably told apart within measurement uncertainty.
  • Only small amounts of reacted sample were recovered, which lowered the X-ray diffraction signal.
  • Modeling suggested iron oxides should have formed in simulated lung fluid, but none were observed. The authors attribute this to the slow dissolution rate.
  • The lifetime estimate does not account for how long any secondary minerals would persist. The authors also say it is uncertain whether lunar dust would produce asbestos-like effects.

Topics

Medical terms in this study, explained in our Lexicon: Olivine, Hydroxyl Radical, Ferrihydrite, Crystal Structure, Asbestos Exposure, Silicosis.

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