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They are usually extracted by pumping large quantities of ammonium sulfate or a similar solution into the ground.

The method extracted a higher percentage of the REEs more quickly than conventional leaching and needed less ammonium sulfate.

The railroad says it also carries fertilizers, including phosphate, urea, ammonium sulfate, ammonium nitrate and anhydrous ammonia.

For example, ammonium sulfate — a principal ingredient of PM2.5 particles — has been associated with ischaemic heart disease, as have particles from diesel engines.

Agriculture has the next biggest impact, contributing to one-fifth of deaths4 — largely through ammonia released by manure and fertilizer use, which combines with sulfate and nitrate in the air to form ammonium sulfate and ammonium nitrate and thus PM2.5.