Atmospheric environment shapes surface reactivity of Fe(0)-doped lunar dust simulant: Potential toxicological implications

The toxicity of lunar dust (LD), anecdotally reported by Apollo astronauts, raises concerns for future missions involving prolonged human presence on the Moon. LD toxicity is thought to involve oxidative stress driven by nanophase metallic iron (np-Fe0), a peculiar feature of LD. In life-supporting lunar habitat, np-Fe0 embedded in the amorphous phases of LD may react with O2 prior to accessing the lung, complicating toxicity assessments. Due to limited availability of real LD samples, toxicological evaluations rely on lunar dust simulants (LDS). A novel Simulant Moon Agglutinate (SMA), composed of a glassy matrix with np-Fe0, was produced and ball milled in an inert atmosphere to expose non-oxidized Fe0 surface centers and to obtain a dust with respirable particle size. Physicochemical properties, oxidative activity, and iron release in simulated body fluids were assessed on selected SMA samples. SMA were aged in air, and the kinetics of free radical generation revealed a strong redox activity that decreased with aging. After an oxidative ageing of 1 month, SMA was still active in generating free radicals, to a higher extent that other LD simulants like JSC-1A-vf, highlighting the key role of np-Fe0 in eliciting LD peculiar reactivity. In vitro tests showed that SMA caused no cell membrane damage, suggesting that LD toxicity mechanisms might involve free radicals and may differ from terrestrial toxic dust, such as quartz.