Particulate matter (PM) induces oxidative stress in vivo, leading to adverse health effects. Oxidative potential (OP) of PM is increasingly studied as a relevant metric for health impact (instead of PM mass concentration) as much of the ambient particle mass do not contribute to PM toxicity. Several assays have been developed to quantify PM oxidative potential and a widely used one is the acellular dithiothreitol (DTT) assay. However in such assays, particles are usually extracted with methanol or Milli-Q water which is unrepresentative of physiological conditions. For this purpose, OPDTT measurements after simulated lung fluids (SLF) extraction, in order to look at the impact of simulated lung fluid constituents, were compared to Milli-Q water extraction measurements. Our major finding is a significant decrease of the OPDTT when the artificial lysosomal fluid (ALF) solution was used. Indeed, ligand compounds are present in the SLF solutions and some induce a decrease of the OP when compared to water extraction. Our results suggest that the effect of ligands and complexation in lining fluids towards PM contaminants probably has been underestimated and should be investigated further.

Dark matter (DM) is the most abundant material in the Universe, but has so far been detected only via its gravitational effects 1 . Several theories suggest that pairs of DM particles can annihilate into a flash of light at γ-ray wavelengths 2 . While γ-ray emission has been observed from environments where DM is expected to accumulate, such as the centre of our Galaxy 3-6 , other high energy sources can create a contaminating astrophysical γ-ray background, thus making DM detection difficult 7,8 . In principle, dwarf galaxies around the Milky Way are a better place to look -they contain a greater fraction of DM with no astrophysical γ-ray background -but they are too distant for γ-rays to have been seen 9 . A range of observational evidence suggests that Omega Centauri (ωCen or NGC 5139), usually classified as the Milky Way's largest globular cluster, is really the core of a captured and stripped dwarf galaxy [10][11][12][13][14] . Importantly, ω Cen is ten times closer to us than known dwarfs. Here we show that not only does ω Cen contain DM with density as high as compact dwarf galaxies, but also that it emits γ-rays with an energy spectrum matching that expected from the annihilation of DM particles with mass 31±4 GeV (68% confidence limit). No astrophysical sources have been found that would otherwise explain ω Cen's γ-ray emission, despite deep multi-wavelength searches [15][16][17] . We anticipate our results to be the starting point for even deeper radio observations of ω Cen. If multi-wavelength searches continue to find no astrophysical explanations, this pristine, nearby clump of DM will become the best place to study DM interactions through forces other than gravity.