Large numbers of Single Use Surgical-type Face Masks, used by the public as personal protective equipment during the 2020–2022 COVID-19 pandemic, have been lost or intentionally discarded and have entered the environment rather than the waste management stream. These masks, made from non-woven polypropylene fibers, will undergo environmental decay which will release fiber fragments as microplastics into the environment. While the photochemical process of the decay of polypropylene polymers (photo-oxidation) is well understood, and while there are numerous studies that investigate mask decay and micro-fiber shedding in laboratory settings, there are no observational data that describe the progress and speed of decay on polypropylene face masks in real-life environmental settings. This paper examines the breakdown of single use surgical-type face masks under natural conditions. Masks from three manufacturers were exposed to natural sunlight over a ten-week period and their state of decay was photographically recorded in situ at weekly intervals. Visible decay accelerated after three weeks, with masks made from thinner spunbond fabric decaying more rapidly. Among same-weight fabric, photo-oxidation affected fabric dyed light blue more than undyed fabric, leading to a total breakdown after six weeks. The results are novel as they demonstrate a differential decay between the spunbonded and the melt-blown fabric, which cracks and breaks down much faster due to thinner fibers of shorter length and the lack of thermal bonding points. The resultant extensive micro-fiber generation was accelerated by external physical forces such as wind. This experiment highlights the fact that municipal agencies have only a narrow window of time to remove stray face masks from the urban environment if micro-fiber pollution is to be prevented.