Certain substances can be considered "environmental" in that they are endogenous to the horse or that they can arise from plants traditionally grazed or harvested as equine feed or are present in equine feed because of contamination or exposure during the cultivation, processing, treatment, storage, or transportation phases. Certain drugs are recognized as substances of human use and could therefore be found in a horse.
Arsenic - 0.3 mcg/ml urine
Caffeine - 100 ng/ml serum or plasma
Cobalt - 50 ppb serum or plasma*
* A level of 25 ppb in serum or plasma will result in the horse being placed on the official veterinarians list until such time as the level drops below the 25 ppb.
Benzoylecgonine - 50 ng/ml urine
Estranediol - 0.045 mcg/ml free + conjugated (5a-estrane-3[BETA],17a-diol), in the urine of male horses, other than geldings
Gamma aminobutyric acid (GABA) - 110 ng/ml in serum or plasma
Hydrocortisone - 1 mcg/ml urine
Methamphetamine - 10 ng/ml urine
Methoxytyramine - 4 mcg/ml, free + conjugated urine
Morphine glucuronides - 50 ng/ml urine
Salicylate salicylic acid - 750 mcg/ml serum or plasma
Theobromine - 2 mcg/ml urine
Tramadol - 50 ng/ml urine
Wash. Admin. Code § 260-70-635