A menstrual pad, or simply a pad, (also known as a sanitary pad, sanitary towel, sanitary napkin or feminine napkin) is an absorbent item worn by women in their underwear when menstruating, bleeding after giving birth, recovering from gynecologic surgery, experiencing a miscarriage or abortion, or in any other situation where it is necessary to absorb a flow of blood from the vagina. A menstrual pad is a type of menstrual hygiene product that is worn externally, unlike tampons and menstrual cups, which are worn inside the vagina. Pads are generally changed by being stripped off the pants and panties, taking out the old pad, sticking the new one on the inside of the panties and pulling them back on. Pads are recommended to be changed every 3–4 hours to avoid certain bacteria that can fester in blood; this time also may differ depending on the kind worn, flow, and the time it is worn. Menstrual pads are made from a range of materials, differing depending on style, country of origin, and brand. The pads are not the same as incontinence pads, which generally have higher absorbency and are worn by those who have urinary incontinence problems. Although menstrual pads are not made for this use, some use them for this purpose. Types Disposable Although producers are generally reluctant to reveal the exact composition of their products, the main materials of disposable menstrual pads are usually bleached rayon (cellulose made from wood pulp), cotton and plastics. In addition, fragrance and antibacterial agents can be included. The plastic parts are the backsheet and polymer powder as an additional powerful absorbent (superabsorbent polymers) that turns into a gel when moistened. Procter & Gamble advertise a proprietary material called infinicel as the core of their napkins. In general, the layering is as follows: "an absorbent core material placed between a flexible liquid-pervious topsheet and a liquid-impervious plastic backsheet that has an adhesive on the outside for attaching the napkin to an undergarment". As is the case with disposable tampons and diapers, recycling is difficult and rarely done due to cost reasons, although proof-of-principle solutions appear to exist. When not dumped in a landfill where the non-biodegradable parts may persist for thousands of years, conventional hygiene products can at best be "thermally recycled" (incinerated). There are several different types of disposable menstrual pads: Panty liner: Designed to absorb daily vaginal discharge, light menstrual flow,