No, we can't be sure. There's a lot of diseases that we don't know the cause of, for example. Cancers, dementia, Alzheimer's, etc. There is a possibility that the rates of those diseases are higher because of plastics. Plastic pollution also accumulates, there was a lot less plastic in the environment a few decades ago. We add more faster than it gets removed, and there could be some threshold after which it becomes more of an issue. We might see the effect a few decades from now. Not only on humans, but it's everywhere in the environment now, affecting all life on earth.