Hi dr, my 10 months old baby swallowed a small sheet of paper of a very old book. the pages were yellowed...is it harmful?

I would not consider an ingestion like this is worth any worry. You can use this as a learning event. Mouthing and ingesting are ways an infant samples/explores the world around them. Make an effort to assure yourself that small objects, fluids, etc. are not available to them if swallowing them is dangerous. Button batteries, rare earth magnets, bugs, screws, are all things we have had to remove

I would not worry about this at all. Paper, even old discolored paper is not harmful generally speaking. While this event was not harmful you can see how quickly small children can get into things and get stuff into their mouths. Keeping a watchful eye and keeping items out of reach is always important. Small items can become choking hazards in an instant. Other things could be very dangerous.