of the hand. Alcohol-free Some hand sanitizer products use agents other than alcohol to kill microorganisms, such as povidone-iodine, benzalkonium chloride or triclosan. The World Health Organization (WHO) and the CDC recommends "persistent" antiseptics for hand sanitizers. Persistent activity is defined as the prolonged or extended antimicrobial activity that prevents or inhibits the proliferation or survival of microorganisms after application of the product. This activity may be demonstrated by sampling a site several minutes or hours after application and demonstrating bacterial antimicrobial effectiveness when compared with a baseline level. This property also has been referred to as "residual activity." Both substantive and nonsubstantive active ingredients can show a persistent effect if they substantially lower the number of bacteria during the wash period. Laboratory studies have shown lingering benzalkonium chloride may be associated with antibiotic resistance in MRSA. Tolerance to alcohol sanitizers may develop in fecal bacteria. Where alcohol sanitizers utilize 62%, or higher, alcohol by weight, only 0.1 to 0.13% of benzalkonium chloride by weight provides equivalent antimicrobial effectiveness. Triclosan has been shown to accumulate in biosolids in the environment, one of the top seven organic contaminants in waste water according to the National Toxicology Program Triclosan leads to various problems with natural biological systems, and triclosan, when combined with chlorine e.g. from tap water, produces dioxins, a probable carcinogen in humans. However, 90–98% of triclosan in waste water biodegrades by both photolytic or natural biological processes or is removed due to sorption in waste water treatment plants. Numerous studies show that only very small traces are detectable in the effluent water that reaches rivers. A series of studies show that photodegradation of triclosan produced 2,4-dichlorophenol and 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD). The 2,4-dichlorophenol itself is known to be biodegradable as well as photodegradable. For DCDD, one of the non-toxic compounds of the dioxin family, a conversion rate of 1% has been reported and estimated half-lives suggest that it is photolabile as well. The formation-decay kinetics of DCDD are also reported by Sanchez-Prado et al. (2006) who claim "transformation of triclosan to toxic dioxins has never been shown and is highly unlikely." Alcohol-free hand sanitizers may be effective immediately while on the skin, but the solutions themselves can become contaminated because alcohol is an in-solution preservative and without it, the alcohol-free solution itself is susceptible to contamination. However, even alcohol-containing hand sanitizers can become contaminated if the alcohol content is not properly controlled