Fine secondary aerosol predicts equine grass sickness onset: a population case-control study in the United Kingdom, 2010–2026

<jats:p>&lt;b&gt;Background:&lt;/b&gt; Equine grass sickness (EGS) is a frequently fatal dysautonomia, but its environmental trigger is unknown. We hypothesised that atmospheric saddle-point stagnation concentrates aerosol and reactive nitrogen over pastures before onset. &lt;b&gt;Objectives:&lt;/b&gt; To test whether atmospheric saddle points are associated with EGS onset and to identify the aerochemical signature and the origin of the dosing air masses. &lt;b&gt;Study design:&lt;/b&gt; Retrospective population case-control study. &lt;b&gt;Methods:&lt;/b&gt; We matched 690 UK EGS cases (2010–2026) 1:10 to displaced controls and atmospheric data (ERA5 winds, CAMS chemistry), detected wind-convergence stagnation events and quantified the exposures preceding each case. Conditional logistic regression assessed associations; a same-location/different-year case-crossover isolated year-specific effects; a 100-day distributed-lag model resolved the timeline. &lt;b&gt;Results:&lt;/b&gt; Fine particulate matter was most strongly associated with onset: PM&lt;sub&gt;2&lt;/sub&gt;.&lt;sub&gt;5&lt;/sub&gt; alone remained significant after mutual adjustment (adjusted OR 1.46, 95% CI 1.07–2.00). The distributed-lag analysis localised the particulate effect to a 50–60-day pre-onset lag (PM&lt;sub&gt;2&lt;/sub&gt;.&lt;sub&gt;5&lt;/sub&gt; peak OR 1.91, 95% CI 1.38–2.64). Gaseous nitric acid, the precursor of ammonium-nitrate aerosol, held in the different-year crossover (OR up to 1.54, 95% CI 1.23–1.92), and in the 2023–2026 high-resolution subset its counterpart ammonia was elevated before onset (OR 2.17, 95% CI 1.66–2.83). &lt;b&gt;Main limitations:&lt;/b&gt; Exposure is ecological, at ~42 km resolution. Reduced-nitrogen precursors could be assessed only in recent high-resolution years, so their year-specificity could not be tested across the full record. Because many atmospheric variables were screened, single-variable associations carry a multiple-comparisons burden and are hypothesis-generating pending prospective replication. &lt;b&gt;Conclusions:&lt;/b&gt; Atmospheric saddle points act as a local lid concentrating fine secondary aerosol over pastures, the association resolving onto fine particulate at a 50–60-day pre-onset lag. Nitric acid and ammonia fit as precursors of that aerosol rather than independent exposures.</jats:p>