Between late 2002 and the first half of 2003, SARS outbreaks occurred in 32 countries and regions, over 8436 SARS cases and 812 deaths were reported by July 5, 2003 while a worldwide alert on SARS was removed (WHO, 2003a). The major mode of transmission of SARS-CoV is through close person contact, in particular, exposure to droplets of respiratory secretions from an infected person (Rota, 2003; Lee, 2003; Tsang, 2003, WHO, 2003b). While a cluster of SARS cases was reported in an apartment block in Hong Kong, wastewater is believed to play a role through droplets containing coronavirus from the wastewater system (WHO, 2003c). SARS-CoV RNA was detectable in urine, stool, and oropharyngeal washing fluid (He et al., 2004; A Study Group of SARS in China, 2004). Liu et al. (2003) reported that the median (range) duration of SARS-CoV excretion in sputa and stools was 21 (14–52) and 27 (16–126) days, respectively. RNA of SARS-CoV was found in the wastewater samples from the Xiao Tang Shan Hospital and 309th Hospital of PLA, which were designated to receive SARS patients in Beijing in 2003 (Wang et al., 2004). These caused serious concern to the disinfection of wastewater of hospitals received SARS patients. However, there have only been a few inactivation studies of SARS-CoV, and much higher concentration of disinfectants was used (Liu, 2003, Tsang, 2003; Li J, 2003).

WHO has warned of the possibility of another outbreak in the winter of 2003, and many infectious disease experts and epidemiologists also predicted new outbreaks in the winter of 2003 or the spring of 2004 (Enserik, 2003; Holden, 2003). These opinions were based mostly on the fact that SARS is spread by the respiratory route and may behave as does influenza. However, there are no new outbreaks as predicted with the exception of some laboratory acquired infected cases and sporadic cases until the spring of 2005 (WHO, 2003d). In part, this is due to the effective prevention methods. But we argue that some unique features of the SARS-CoV such as short persistence in the environment and low resistance to disinfectants may also explain why there are no further outbreaks.

The purpose of this paper is to explore conditions that favored the persistence of SARS-CoV in different environments and the effect of disinfectants in inactivating SARS-CoV, Escherichia coli and f₂ phage.

SARS is a new infectious disease caused by a new coronavirus (Rota, 2003;Tsang, 2003b; Cyranoski, 2003). Although there is prevailing belief of another SARS outbreak (Enserik, 2003; Holden, 2003), only two laboratory acquired infections in Singapore and Taiwan, nine cases in China (two were suspected lab-cross infection from the Chinese National Institute of Virology in Beijing, Center for Disease Control, and the others were in contact with these two cases), and four separate cases in Guangzhou (all the four cases were in contact with animals like civet cat), have been reported so far (WHO, 2004).

SARS-CoV does not appear to persist in vitro environments, as was believed previously. However, SARS-CoV may persist longer at relative low temperatures, and will do so in PBS and urine. This may relate to the fact that such fluids contain salts, which maintain osmotic pressure of virus needs for persistence. Such characteristics of SARS-CoV found in our experiment are basically similar to other reports. A report in China's Science and Technology Daily on 4 June, 2003 noted that the researchers of the Academy of Military Medical Sciences, and Center for Disease Control and Prevention of China found that at 24 °C, SARS-CoV may persist for 5 days in sputum and feces, 19 days in urine and 3 days on the surfaces of objects (Liu, 2003). Tsang et al. (2003) also reported that SARS-CoV might survive for 1–4 days in feces and 0.5–3 days on the surfaces of objects.

The major transmission mode of SARS-CoV is via close human contact, in particular, exposure to droplets of respiratory secretions from an infected person (Lee, 2003). However, in a cluster of SARS cases in a Hong Kong apartment block, investigators found that SARS-CoV nucleic acid can also be detected from stools of the patients (Cyranoski, 2003). So there is a great concern on the disinfection of SARS-CoV in patient excrements and wastewater.

Because wastewater always contains high number of potentially pathogenic bacteria and viruses, some non-pathogenic organisms are often used as indicators in studies on disinfection effect of disinfectants on pathogenic organisms in wastewater. The most commonly used indicators include E. coli, f₂ phage, MS₂ phage and poliovirus. Tree et al. (2003) reported that inactivation (>5 log₁₀ units) of E. coli and Enterococcus faecalis was rapid and complete but that there was poor inactivation (0.2–1.0 log₁₀ unit) of F⁺-specific RNA bacteriophage (MS₂) at all the three chlorine concentrations (8, 16 and 30 mg/L). However, seeded poliovirus was significantly more susceptible (2.8 log₁₀ units) to inactivation by chlorine than was the MS₂ phage. Shah PC. and McCamish J. (1972) found that relative chlorine resistance of f₂ phage (F⁺-RNA) is obviously stronger than that of poliovirus and T₂ phage. Tyrrell et al. (1995) found that the resistance of F⁺-phage against chlorine was as 10 times as that of E. coli and Enterococcus in disinfection of secondary effluent from water treatment plants. After comparing disinfection effects of different microorganism in different waters, Havelaa (1987) and Sebastiani (1989) concluded that phages, F⁺-phage in particular, is most suitable for serving as an indicator microorganism for evaluating disinfection effects. Therefore, we chose f₂ phage and E. coli 8099 as the indicator microorganisms for research on the inactivation of SARS-CoV by disinfectants.

So far there have been few reports on the persistence of SARS-CoV in the environment or resistance to conventional disinfectants. Bao et al. (2003) reported that the infectiousness of SARS-CoV was maintained at least 10 days at 4 °C; infection titer was decreased from 7.5 TCID₅₀ to 3.2 TCID₅₀ within 5 days at room temperature; the virus was sensitive to heating, and could be completely inactivated either by being heated for 30 min at 56 °C or for 5 min at 70 °C. Rabenau et al. (2004) studied the stability of SARS-CoV under different conditions, both in suspension and dried on surfaces, in comparison with human coronavirus HCoV-229E. In suspension, HCoV-229E gradually lost its infectivity completely while SARS-CoV retained its infectivity for up to 9 days; in the dried state, persistence times were 24 h versus 6 days. Thermal inactivation at 56 °C was highly effective in the absence of protein. Duan et al. (2003) reported that SARS coronavirus under the testing condition could survive in the serum, 1:20 diluted sputum and feces for at least 96 h, whereas it could remain alive in urine for at least 72 h with a low level of infectivity. The persistence on the surfaces of eight different materials and in water was comparable, revealing reduction of infectivity after 72–96 h exposure. Viruses remained stable at 4 °C, at room temperature (20 °C) and at 37 °C for at least 2 h without remarkable change in the infectious ability in cells, but were converted to be non-infectious after 90-, 60- and 30-min exposure at 56, 67 and 75 °C, respectively. Irradiation of UV for 60 min on the virus in culture medium resulted in the destruction of viral infectivity at an undetectable level. One article in China's Science and Technology Daily on June 4, 2003 states that SARS-CoV in feces or urine could be inactivated within a few minutes by 500–1000 mg/L of chlorine or peracetic acid, while the virus could also be killed with ultraviolet radiation or heating for 30 min (Liu, 2003). Tsang (2003) reported that SARS-CoV could be inactivated in 75% alcohol, 2% hydroxybenzene solution, 500 mg/L sodium hyperchlorine or detergents for 5 min, which suggests that the virus is sensitive to all sorts of disinfectants. Li J et al. (2003) reported that SARS-CoV could also be inactivated more than 5 log₁₀ units within 60 s in 80% solution of a compound disinfectant comprising 1700–1900 mg/L of chlorhexidine acetate and 1000 mg/Lof nano-zinc oxide.

As shown by the above results, SARS-CoV is sensitive to either environmental factors or disinfectants. However, the concentration of disinfectants was too high to apply in water or wastewater disinfection practice. We observed the inactivation effect of chlorine and chlorine dioxide on SARS-CoV with common concentrations and persistence in different wastewater. It is found that SARS-CoV was easier to inactivate by chlorine or chlorine dioxide than E. coli and f₂ phage in wastewater, and its infectivity in environment is easy to lose. The characteristics of SARS-CoV could be confirmed indirectly by another experiment (Wang, 2004). Only 1% of SARS-CoV seeded in wastewater could be recovered by a type of electropositive filter media particle, which worked well with the recovery of many types of enteroviruses in the previous study (Li et al., 1998). We believe that the reason for the low recovery of SARS-CoV may be due to its weak resistance and high sensitivity to the environment factors as well as damage to the virus during concentration procedures.

Chlorine has long been used as a simple and economic method for disinfection worldwide to ensure the safety of drinking water, however, the continued use of chlorine for the disinfection of potable water supplies comes under greater scrutiny owing to the potential health hazards posed by the resulting chlorinated hydrocarbons, including trihalomethanes and haloacetic acids (Rook, 1974). Furthermore, chlorine is a poor disinfectant above pH 8 and in some cases a poor virucide at pH 5 and 6 (Taylor and Butler, 1982a). Chlorine dioxide is used as an alternative disinfectant because it does not form halogenated by-products, and is better than or equivalent to chlorine in the bactericidal effects and more remarkable than that of chlorine as a virucide in a wider pH range (Li et al., 1996, Li et al., 2004, Taylor and Butler, 1982b, Huang et al., 1997). However, Tsai and Lin (1999) reported that hypochlorite was better in inactivating E. coli in hospital wastewater and sludge than chlorine dioxide. This study also suggested that in terms of inactivating E. coli, f₂ phage and SARS-CoV, chlorine is better than chlorine dioxide. The reason for this is not very clear but may probably be related to excessive content of reducing substances or organisms. Therefore, in terms of economics or security, chlorine is the best choice for disinfection of hospital wastewater.

Above all, SARS-CoV can only persist as infectious particles for a very short time in vitro environments and is highly sensitive to conventional disinfectants. In addition, large amount of various disinfectants were used for environment disinfection in China's mainland during the SARS epidemic in 2003, the effect of high temperature in summertime, and stick control and management, except animal-to-human transmission or cross infection within labs, there is little possibility for another outbreak caused by SARS-CoV from environmental sources.