When you breathe out through a mask with an exhalation valve, it’s only a one-way protection. The air you are breathing out is not filtered thus making the air you exhale contaminated and affecting those around you.
Surgical masks are not designed or certified to prevent the inhalation of nanoparticles. These particles are not visible to the naked eye, but may still be capable of causing infection.
More layers are not better. Each layer adds additional restriction, making it harder for the air to go through the filter media within the mask.