These labels are also seen on magazine subscription renewals, allowing customers to re-subscribe to the magazine with an easy peel-and-stick label sent back. Also, as the retained label is adhesive free it prevents customers from re-applying the cheaper-priced labels to premium products. Smart labels have RFID chips embedded under the label stock. Blockout labels are not see-through at all, concealing what lies underneath with a strong gray adhesive. Radioactive labels. The use of radioactive isotopes of chemical elements, such as carbon-14, to allow the in vivo tracking of chemical compounds. Laser or printer labels are generally die cut on 8.5" x 11" (US letter) or A4 sized sheets, and come in many different shapes, sizes, formats and materials. Laser label material is a nonporous stock made to withstand the intense heat of laser printers and copiers. A drawback of laser labels is that the entire sheet needs to be printed before any labels are used; once labels have been removed the sheet cannot be put through the printer again without damaging the printing mechanism. Inkjet label material is a porous stock made to accept ink and dye from an inkjet printer. One of the more modern inkjet label material stocks is waterproof printable inkjet material commonly used for soap or shower gel containers. Security labels are used for anti-counterfeiting, brand protection, tamper-evident seals and anti-pilferage seals. These combine a number of overt and covert features to make reproduction difficult. The use of security printing, holography, embossing, barcodes, RFID chips, custom printing and weak (or weakened) backings are common. They are used for authentication, theft reduction, and protection against counterfeit and are commonly used on ID cards, credit cards, packaging, and products from CDs to electronics to clothing. Antimicrobial labels. With the growth in hospital-acquired infections such as MRSA and E-Coli, the use of antimicrobial labels in infection-sensitive areas of hospitals are helping in combating these types of microbes. Fold-out labels, also known as booklet, multi-page, multi-layer, or extended labels, or lablets (combined label + leaflet). Where the pack is not large enough for a single label to carry all the required information, fold-out labels are often preferred to separate leaflets, which can easily be lost. These labels are frequently seen on agricultural chemicals and consumer pharmaceuticals. Barcode labels A large proportion of labels produced today carry barcodes, either for product identification, for traceability in items such as freight packages, and