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.

(1) The following substances are permissible in test samples up to the stated concentrations:

Arsenic - 0.3 mcg/ml urine

Caffeine - 100 ng/ml serum or plasma

Cobalt - 50 ppb serum or plasma*

Benzoylecgonine - 50 ng/ml urine

Estranediol - 0.045 mcg/ml free + conjugated (5a-estrane-3β,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

(2) If a preponderance of evidence presented shows that a positive test is the result of environmental substance or inadvertent exposure due to human drug use, that evidence should be considered as a mitigating factor in any disciplinary action taken against the trainer.