Why only the chewing cat is at risk
Peace lilies contain calcium oxalate crystals that must be physically released by chewing or tearing the plant. Your other two cats didn't ingest any plant material, so their mouths and digestive tracts remain untouched by the irritants. Unlike airborne allergens or contagious toxins, these crystals don't spread through proximity or shared space — they require direct mechanical action to become a problem.
Watch the observing cats for any unusual lip-licking or pawing at their faces, but these would be stress behaviors from witnessing their companion's distress rather than chemical irritation. The 4 kg cat who actually chewed the leaf needs your full attention right now, as their mouth is likely already burning from the microscopic needle-like raphides.
What to do immediately for the affected cat
Gently flush your cat's mouth with cool water using a syringe or soaked cloth to dislodge embedded crystals. Avoid forcing water if they're panicking, as this could cause aspiration. The goal is to rinse without adding stress while you prepare for transport. Offer small ice chips to numb discomfort temporarily, but don't let them swallow large chunks that might upset their stomach further.
Check their gums for redness or swelling, particularly along the tongue and inner cheeks where the crystals penetrate deepest. Even if symptoms seem mild at first, the delayed swelling can make breathing difficult in small cats, so don't wait to see if it worsens. Your 4 kg size means faster metabolic effects than a larger animal would experience.
Why emergency care can't wait
Small cats dehydrate rapidly from excessive drooling, and their narrow airways are more vulnerable to swelling obstruction. At 4 kg, your cat has less body mass to buffer against systemic inflammation. While peace lilies aren't typically fatal, the pain and secondary complications (like refusal to eat due to mouth sores) hit lightweight pets harder.
Vets often administer pain relief and anti-inflammatory medications to prevent esophageal spasms and allow comfortable swallowing. They might run blood work to check kidney values if significant plant material was swallowed, though calcium oxalate toxicity primarily causes local rather than systemic damage in cats. Hospitalization for IV fluids is sometimes needed if drooling leads to electrolyte imbalances.
Separating cats during the crisis
While the other cats aren't chemically affected, their curiosity could lead them to investigate the chewed plant or vomit from the sick cat. Confine them to a different room with fresh water and litter boxes while you handle the emergency. Cats often mimic symptoms when stressed, so don't mistake their concerned behavior for actual toxicity.
Dispose of the damaged leaf securely where no pets can reach it, and consider moving the entire plant to an inaccessible location. Peace lilies tempt cats with their broad, chewable leaves, and one cat's negative experience won't necessarily deter others from sampling it later. Monitor all pets for 24 hours, but focus veterinary attention solely on the cat who made direct contact.
The step-by-step response, from the first minute to the vet visit, is in our Peace Lily Plant Poisoning in Dogs and Cats emergency guide.