I am unsure of the exact effects but cat urine as amonia in it and is toxic. Thats why they tell you that if you have a cat and are pregnant then you should wear a mask or have someone else clean the litter for you because it can affect the development of the fetus. I would think that if the cats were indoor then there would be a definate environmental hazard for your grandson as the litter box would be within the home. If the cats were outdoors, then perhaps there would be less concern unless they were dirty cats Also, exposure to a significant amount of animal dander could trigger an asthma attack in a young child, or at least it did for my younger sister who has asthma. I am not a medical practitioner I am just letting you know what I have been told or witnessed. Hope this helps and I hope your Great-Grandson is ok.
The reason pregnant women are told not to clean the cat litter is because cat (and dog) faeces can contain toxoplasma, which is a bacteria which caused toxoplasmosis - if a pregnant woman was to contract this, the fetus could be harmed, particularly it causes brain damage in the fetus.
The urine isn't a huge issue for your grandson, however the cat and dog hair could be contributing to his asthma.
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