hypoallergenic. A number of materials are used, with varying strengths and weaknesses. Surgical stainless steel, niobium and titanium are commonly used metals, with titanium the least likely to cause allergic reaction of the three. Platinum and palladium are also safe alternatives, even in fresh piercings. Initial piercings should never be done with gold of any grade, as gold is mixed with other metals, and sterling silver is not a good alternative in a piercing, as it may cause allergies in initial piercings and will tarnish in piercings of any age. An additional risk for allergic reaction may arise when the stud or clasp of jewellery is made from a different metal than the primary piece. Body piercing jewellery is measured by thickness and diameter/length. Most countries use millimeters. In the US, the Brown & Sharpe AWG gauging system is used, which assigns lower numbers to thicker middles. 00 gauge is , while 20 gauge is . For discussion of gauges, see: "Body jewelry sizes." Piercing tools Permanent body piercings are performed by creating an opening in the body using a sharp object through the area to be pierced. This can either be done by puncturing an opening using a needle (usually a hollow medical needle) or scalpel or by removing tissue, either with a dermal punch or through scalpelling. Tools used in body piercing include: The piercing needle The standard method in the United States involves making an opening using a beveled-tip hollow medical needle, which is available in different lengths, gauges and even shapes. While straight needles are useful for many body parts, curved needles are manufactured for areas where straight needles are not ideal. The needle selected is typically the same gauge (or sometimes larger as with cartilage piercings) as the initial jewellery to be worn, with higher gauges indicating thinner needles. The needle is inserted into the body part being pierced, frequently by hand but sometimes with the aid of a needle holder or pusher. While the needle is still in the body, the initial jewellery to be worn in the piercing is pushed through the opening, following the back of the needle. Jewellery is often inserted into the hollow end of a needle, so that as the needle pulls through the jewellery is left behind. The indwelling cannula Outside of the United States, many piercers use a needle containing a cannula (or catheter), a hollow plastic