Camera overlays?

Hello,

I'm not sure if I'm writing this in the correct board or should have asked in "Unity Answers". Needless to say:

I'm not sure if there is something in Unity or on the assets store for this, but some games have had overlays often on cameras (Sometimes with blood when injured) and rain droplets on the screen (Although not particularly realistic in most cases) generally first person.

Does anyone have a solution for either putting an overlay on the camera, whether this be slightly broken glass or rain droplet effect. As I wanted to make it seem like my character will have a gas mask on similar to that of Metro2033.

Alpha texture on the camera or an image effect?

Example:

Thanks for your time!