Patients presenting to the Department of Endodontics, School of Dental Medicine, University of Pennsylvania for evaluation and routine endodontic treatment of infected, necrotic teeth with chronic apical periodontitis will be asked to take part in the study if they meet the inclusion criteria and volunteer to participate. After eligibility of the patient is confirmed, the patient will be assigned to treatments through the process of drawing lots from a box that was maintained in a locked cabinet. Before treatment, patients will be thoroughly informed about the nature, potential risks and alternatives of the study as well as the root canal treatment. Patients will be presented with a written consent form regarding the above mentioned study characteristics as well as the regular consent forms for the root canal therapy, including the consent form for endodontic treatement, acknowledgement of privacy practices and a patient understanding and informed consent form. Briefly, the patient will be anesthetized and the tooth isolated with rubber dam. 30% H2O2 followed by 3% NaOCl will be used to disinfect the tooth and the rubber dam. The removal of caries and the endodontic access will be carried out by sterile high-speed carbide burs.After access preparation with sterile burs and sterile saline irrigation, thermoplastic gutta percha was placed to temporarily block the orifice. The field, including the pulp chamber, is cleaned and disinfected as described previously. NaOCl is neutralized with 10% sodium thiosulfate. Contamination control sample (S0) will be taken from the internal cavosurface angle where the paper points will accidentally touch during sampling.After initial access to the root canal orifices, working length will be measured and a bacteriological sample will be taken from the targeted canals (S1). Sterile paper points will be placed into the canal, allowed to saturate and then transferred to a vial containing liquid dental transport media (LDT). For NaOCl group (Positive control), canals will be instrumented up to size 25/0.04 taper using 2mL of 3% NaOCl in between files. For Ferumoxytol/H2O2 group, canals will be instrumented up to size 25/0.04 taper using 2mL of a mixture of 6 mg/ml of Ferumoxytol with 3% H2O2. For saline only group, canals will be instrumented up to size 25/0.04 taper using 2mL of saline. When the final 25/0.04 taper apical size is reached, a second bacterial sample will be taken (S2). Before all samplings, sodium hypochlorite, Feramehe/ H2O2 and Saline. Canal contents will be deactivated with sodium thiosulphate for NaOCl,and saline wash will be used for Fer/H2O2 and saline treatments. A wash step with 1 mL saline was done to wash the deactivating solution, and paper points were used to dry the canals. A second bacterial sample was taken (S2) by placing LDT inside the canal, agitating it with 25/0.02 Hedstrom hand file, followed by absorbing the content with 2 paper points placed in the canal for 30 seconds each. The paper points will be placed inside a tube containing LDT. An additional step was included for all the test groups to evaluate if further irrigation after (S2) will lead to more reduction of bacterial counts inside the root canal system. This could inform future experiments evaluating the possibility of synergistic antimicrobial effects for the sequential Fer/H2O2 and NaOCl treatment. All canals in all groups were irrigated with 2 mL of 3% NaOCl (1 min), followed by ultrasonic irrigant activation for 30 seconds. This step was repeated once again making the total activation time 1 min and the total contact time of the irrigant 3 min. Upon completion of irrigation, a third bacterial sample (S3) was taken following the deactivation, washing, drying, and sampling steps as described previously. The remaining treatment sequences of the routine root canal therapy will be carried out after these procedures including further root-end enlargement and final routine irrigation protocol. The root canals will be dried with paper points, a medication (calcium hydroxide) will be placed and the teeth sealed with a temporary restoration. Patients will return after one to four weeks for completion of the root filling. For the any of the groups, the treatment procedures carried out during this investigation do not differ from the standard root canal treatment protocol with the exception of additional irrigation step with the experimental solution and the bacteriologic sampling procedures. The paper points used to take the bacteriological sampling will be transferred to the microbiology laboratory using a vial containing 1 ml of LDT. The laboratory procedures will be performed at the University of Pennsylvania Leon Levy Oral Health Sciences Building of the School of Dental Medicine in the Microbiology Laboratory Vial labels will contain information on tooth number, sample number (S0-S1-S2-S3) and the experimental group. The samples will be diluted and plated in culture plates. The culture plates will be incubated at 37°C in an anaerobic glove box containing 5% hydrogen, 5% CO2 and balance N2 for 5 days. After incubation the number of colony forming units will be determined by using a stereoscope. ANOVA and Students t-test will be used for statistical analysis.