Urinal deodorizer block
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Urinal deodorizer blocks (also called urinal cakes, para blocks, urinal mints, urinal pucks, urinal peons (pee-ons), urinal biscuits, or urinal cookies) are the small disinfectant blocks found in urinals.
The purpose of these blocks is to disinfect and reduce or mask the odor of restroom urinals. They are placed above the urinal drain (often in the confines of a small plastic trap "urinal screen" that prevents their loss down the drain when they dissolve to a small size).
Appearance and composition 
Urinal deodorizer blocks are tablets (usually cylindrical in shape) used to mask bad odors in washrooms. The chemicals composing the block may vary. The original formulations were of naphthalene and later para-dichlorobenzene (pDCB), both now known to be hazardous to health by inhalation. Most urinal blocks are now para-dichlorobenzene-free. In some areas, the use of para-dichlorobenzene toilet blocks has been banned; in other areas para-dichlorobenzene blocks or "para blocks" are still used.
Para-dichlorobenzene and naphthalene blocks do not actually dissolve in water or urine, but instead sublimate into the air, creating a strong sickly-sweet odor that masks the smell of urine. Urinal-deodorizer companies now produce water-soluble versions made from surfactants, which offer some active cleaning efficacy. The new water-soluble blocks improve the cleaning of the pipes to remove some of the cause of odor problems. Some recent formulations also include bacterial strains which, coupled with the surfactant cleaning power, can more completely eradicate odors and blockages caused by the buildup of solids in the traps and pipes. Some manufacturers claim that these "biological blocks" can enable completely no-flush waterless urinals to be fitted.
Alternative toilet blocks may release pleasant odor, but also support cleaning and disinfecting of toilet bowls and urinals, in contrast to para-dichlorobenzene, which does not have cleaning or disinfecting properties. The block's functionality is enhanced by saturating it with a scenting compound and quaternary ammonium compounds. They come in a variety of shapes and sizes, and weigh from about 20 grams up to about 100 grams when new.