Why is it safe?
Cardboard is not toxic. Cats are strongly attracted to enclosed spaces such as boxes, they provide security, thermal insulation, and a vantage point, all of which are behavioral needs for the species. Normal interaction carries essentially no risk. The minimal concerns are damp or moldy cardboard (fungal exposure) and compulsive chewing with ingestion in PICA-prone cats.
Symptoms
Normal interaction: no symptoms. Large cardboard ingestion: possible vomiting or mild GI upset. Moldy cardboard: possible respiratory irritation or, in extreme cases, mycotoxin exposure.
What To Do
No action needed for normal box interaction. Discard boxes that are wet, moldy, or that were shipped with fumigation chemicals. Monitor if a cat is ingesting large amounts compulsively.
Notes
Research by Mehrkam and Wynne (2014) and subsequent studies confirm that cats consistently prefer boxes over other forms of environmental enrichment in novel and stressful settings. A cardboard box is one of the most effective, zero-cost enrichment options available. Boxes from commercial shipments that have been fumigated for international shipping should be aired out before offering.
Sources
→ Mehrkam & Wynne (2014) Applied Animal Behaviour Science — sciencedirect.com
→ International Cat Care — icatcare.org
→ Cornell Feline Health Center — vet.cornell.edu