I.G.
You can buy organic wool rugs or rugs made from other kinds of natural fiber, they are just pretty pricey.
Polypropylene itself is not considered toxic. It is widely used for clothing, packaging materials (everything that is marked with a 5 in the recycle symbol), even surgical suture material. BUT rugs made out of PP are often treated with fire retardants and other chemicals (everything from dye to herbicides that protect the product during storage and transport) which can evaporate (hmm, that new carpet smell - a mix of several toxins and carcinogens) and leach out.
If you are concerned you should contact the manufacturer and ask if there are any toxicology results for their products or what they are doing to make their product more environmentally and health friendly. Many companies have these data and simply don't publish them in easily accessible places.
Good luck.