Copyright holders have been investigating technological solutions to prevent distribution of copyrighted materials in peer-to-peer file sharing networks. A particularly popular technique consists in "poisoning" a specific item (movie, song, or software title) by injecting a massive number of decoys into the peer-to-peer network, to reduce the availability of the targeted item. In addition to poisoning, pollution, that is, the accidental injection of unusable copies of files in the network, also decreases content availability. In this paper, we attempt to provide a first step toward understanding the differences between pollution and poisoning, and their respective impact on content availability in peer-to-peer file sharing networks. To that effect, we conduct a measurement study of content availability in the four most popular peer-to-peer file sharing networks, in the absence of poisoning, and then simulate different poisoning strategies on the measured data to evaluate their potential impact. We exhibit a strong correlation between content availability and topological properties of the underlying peer-to-peer network, and show that the injection of a small number of decoys can seriously impact the users' perception of content availability.