The aim of this study is to verify, through bacteriological analysis, the efficacy of reducing the bacterial load referred to the presence of Streptococcus Mutans in carious lesions of viable deciduous molars by application of gaseous ozone or ozonized gel.

Patients who meet the eligibility criteria and for whom the legal guardians have signed the informed consent, will undergo treatment of the carious lesion with gaseous ozone or ozonized gel. After selective removal of dentin, a bacterial sample will be taken from the carious surface with a sterile paper cone at time T0; then ozone will be applied for 30 seconds, the surface will be washed with sterile saline and dried with air, a second sample will be taken at time T2; finally, ozone will be applied for another 30 seconds, the surface will be washed with sterile saline and dried with air, a third sample will be taken at time T2.

Patients are then randomised into two groups:

Test group: gaseous ozone (Healozone X4) is applied to the carious lesion for 30 seconds and then for another 30 seconds.

Control group: ozone gel (GeliO3) will be applied to the carious lesion for 30 seconds and then for another 30 seconds.

Changes in the following indices will be assessed at the first visit and when the samples are taken at T0, T1, T2: compliance assessment (FLACC scale), sensitivity test (Schiff Air Index), assessment of lesion severity and extent (ICDAS), assessment of gingival inflammation (GI), assessment of plaque index (PI), assessment of severity of enamel erosion (Bewe index).

Inclusion Criteria:

• Patients suffering from carious lesion on viable deciduous molar
• patients' compliance

Exclusion Criteria:

• patients with early loss of deciduous molars for carious lesions or orthodontic reasons
• non-viable deciduous molars
• deciduous molars with traumatic lesions

Data will be available upon motivated request to the corresponding authors.