The bacteria present in infected root canals include a restricted group of species compared with the total flora of the oral cavity. The bacterial species in root canals may vary, and the number of bacterial cells recovered are between < 10 2 to > 10 8[1].

Black-pigmented bacilli (BPB) have frequently been isolated from root canals along with Streptococci in 70%- 82% of the root canals of deciduous teeth with pulp necrosis[2]. The anaerobiosis is broken when the canal is opened and biomechanical treatment eliminates bacteria as well as deprives the canal of nutrients and interferes with bacterial interactions[1].

Sodium hypochlorite (NaOCl) is reported to kill the target microorganisms in seconds, even at low concentrations. Continuous irrigation and time are important factors for the effectiveness of hypochlorite[3]. It has been shown that using 0.5% or 5% NaOCl, with or without EDTA for irrigation, resulted in considerable reduction of bacterial counts in the canal when compared with irrigation with saline but it was difficult to render the canals completely free from bacteria, even after repeated sessions[4].

The disadvantages of NaOCl include the unpleasant taste, toxicity, and its inability to remove the smear layer by itself, as it dissolves only organic material[5]. The limited antimicrobial effectiveness of NaOCl in vivo is also disappointing because of poor penetration to the most peripheral parts of the root-canal system such as fins, anastomoses, apical canal, lateral canals, and dentin canals. Also, the presence of organic materials like exudate from the periapical area, pulp tissue, dentin collagen, and microbial biomass counteract the efficacy of NaOCl[3]. Recently, it has been shown by in vitro studies that long-term exposure of dentin to a high concentration sodium hypochlorite can have a detrimental effect on dentin elasticity and flexural strength increasing the risk of vertical root fracture[6][7]. However, use of hypochlorite as the final rinse following EDTA or citric acid (CA) rapidly produces severe erosion of the canal-wall dentin and should probably be avoided[8].

Edward Fisch (1932) was the first to use ozone to control infection in regular dental practice. Ozone(O3) is energized form of oxygen which is unstable and dissociates readily back to oxygen (O2) and singlet oxygen (O) which is a strong oxidizing agent[9].

There are three basic forms of ozone application.

1) Ozone gas

2) Ozonated water

3) Ozonated oil.

These forms of application are used singly or in combination to treat dental disease. Ozone gas has a half life of about 5-30 min and Ozonated water has a life span of about 10 hrs if the water is kept cold[10].

It has been proved that ozonated water (0.5–4 mg/l) was highly effective in killing both gram-positive and gram-negative oral microorganisms. Among them, the gram-negative bacteria, such as the endodontopathic bacterium P. endodontalis and the periodontopathic bacterium P. gingivalis, were substantially more sensitive to ozonated water than the gram-positive oral streptococci[11].

The advantages of ozone in the aqueous phase are its potency, ease of handling, lack of mutagenicity, rapid microbicidal effects[12]. Also, aqueous ozone fulfils optimal cell biological characteristics in terms of biocompatibility for oral application.

Therefore the purpose of this study is to evaluate and compare the anti- microbial efficacy of sodium hypochlorite and aqueous ozone against the bacteria found in the non- vital tooth which are- anaerobes like black pigmented bacilli (P.Gingivalis, Prevotella, P. Micros), E. Faecalis and aerobes- streptococcus species. this study was planned with hypotheses of significant difference in anti- microbial efficacy of aqueous ozone in comparison to NaOCl as root canal irrigant in non-vital primary teeth