Effects of low intermittent concentrations of nitrous oxide on the developing rat fetus

Five groups of 12 gravid female rats were exposed, during their entire gestation, intermittently for 6 h a day, 5 days per week, to 0.5% (5000 p.p.m.), 0.1% (1000 p.p.m.), 0.05% (500 p.p.m.) and 0.025% (250 p.p.m.) nitrous oxide-air mixtures (v/v), respectively. After killing the gravid rats on the 19th day of their pregnancies the numbers of corpora lutea of pregnancy were counted. Each uterus was examined for placentation sites and embryonic remnants and a detailed examination of the fetuses was undertaken. A significant reduction in mean litter size occurred only in the group exposed to 0.5% nitrous oxide in air (v/v). The mean litter sizes of the control and other exposed groups were comparable. No signs of fetal resorption or skeletal malformation were found in any group. The threshold of pollution for reduction in litter size has been shown in this study to lie between 0.1% and 0.5% nitrous oxide in air (v/v). A previous study on continuous exposure reported the threshold pollution to lie between 0.05% and 0.1% nitrous oxide in air (v/v). This investigation confirms that intermittent exposure to nitrous oxide-air mixtures increases the threshold for reproductive effects in rats.

Effects of low concentrations of nitrous oxide on rat fetuses

The long-term inhalation of air polluted with nitrous oxide has been implicated in spontaneous abortions and fetal abnormalities in operating room personnel. Studies with 1% and 0.5% N2O in air have confirmed the adverse effects of dilute concentrations of N2O in pregnant rats. The present study was designed to determine the threshold concentration of nitrous oxide necessary to produce adverse responses in pregnant rats. Wistar albino rats were mated in an environmental chamber and conception confirmed by vaginal smears. Immediately thereafter, the rats were randomly divided into control and experimental groups. The control rats were exposed for 19 days to oil-free compressed air in an environmental chamber at a flow rate of 8.5 air changes per hour. Experimental animals were exposed continuously under identical conditions to concentrations of 0.0, 0.1%, 0.5%, or 0.025% N2O. On day 19 the gravid rats were killed. The corpora lutea of pregnancy were counted and related to the number of fetuses in the uterus. Fetuses were fixed, cleared, and their skeletons stained with alizarin red. Litter size, frequency of fetal resorption, and fetal crown rump measurements were significantly different following exposure to 0.1% N2O than they were in control rats or in rats exposed to 0.05% or 0.025% nitrous oxide, neither of which had any fetal effects. The suggested threshold level of 30 ppm (0.03%) N2O in humans may be unrealistically low.

Dental caries status of Chinese children in Johannesburg, South Africa

The purpose of this study was to determine the dental health status of a representative sample of Johannesburg Chinese schoolchildren, all 250 attending the only Chinese school in the city. In 18 preschoolchildren, 3--5 years old, 16.7% were caries-free, mean dmft was 7.1 +/- 5.8 and labial caries was present in 33.3%. In 165 primary schoolchildren aged 5--16 years, the mean dmft was 590 +/- 3.2 with 20% of the primary dentition caries-free and the mean DMFT was 2.4 +/- 1.9 4.8% of the primary schoolchildren were caries-free. In 67 high school pupils of 11--17 years, 4.5% were caries-free and the mean DMFT score was 7.1 +/- 3.9. Caries prevalences among the Chinese were similar to corresponding groups of children of Chinese immigrants in the United Kingdom and Malaysia.

Effect of the chronic administration of nitrous oxide 0.5% to gravid rats

Twenty-four gravid Wistar rats (12 experimental and 12 control) were studied. From day 1 to day 19 of gestation the experimental group was exposed day and night to nitrous oxide 0.5%, while the control group was exposed to air. On day 19 the rats were killed and a detailed examination of the uterus, ovaries and fetuses was undertaken. The fetuses were fixed, cleared and stained with alizarin red, examined for skeletal anomalies and their crown-rump lengths measured. There was a significant decrease in litter size of the dams exposed to nitrous oxide as compared with control (P less than 0.001). Fetal resorption occurred in 10 of the 12 exposed dams. Skeletal anomalies were observed in 9% of the fetuses in the experimental group and there was an overall reduction in their size. None of the above changes were observed in the control.

Intermittent exposure of gravid rats to 1% nitrous oxide and the effect on the postnatal growth of their offspring

Three groups of pregnant rats were exposed to 1% nitrous oxide 6 hours a day, 5 days per week for the first, first and second, and all 3 weeks of gestation. The offspring were weighed and measured at weekly intervals. The following variables were evaluated in control and experimental groups: litter size, body weight, tail length and body length. Litter sizes in animals exposed for the first and second weeks of gestation were significantly smaller than the controls. A two-way nested statistical analysis of the data revealed that over an 8-week period the experimental rats were significantly smaller than the control rats of the same age. Analysis of the 95% confidence intervals revealed no particular pattern related to the timing of the exposure to nitrous oxide.

Effect of intermittent exposure to a low concentration of nitrous oxide on haemopoiesis in rats

Wistar strain albino rats were exposed to 1% nitrous oxide in air for periods ranging from 1 week to 6 months. Their peripheral blood and bone marrow were examined for evidence of alterations in haemopoiesis. Apart from a transient polycythaemia during the first week no change in haemopoiesis was found.