Heinrich et al (1995) exposed female rats by whole body inhalation to ultrafine TiO2 (80% anatase: 20% rutile) at an average concentration of 10 mg/m3 for 24 months followed by 6 months without exposure. The particle size of the TiO2 used ranged from 15 to 40 nm with a MMAD of 0.8 mm (agglomerates of ultrafine particles). Statistically significant increases in tumours vs. controls were observed in rats from this study (benign keratinising cystic squamous-cell tumours, adenocarcinomas, squamous-cell carcinomas, and adenomas). Exposure of female mice to ultrafine TiO2 under the same conditions as for rats resulted in a significantly decreased lifespan at an inhalation concentration of approximately 10 mg/m3 (Heinrich et al, 1995). However, tumour rates were not statistically increased over prevalence in controls.
Groups of female Wistar rats were exposed by inhalation 17 hr/day, 5 days/week to 6 mg/m3 Printex CB particles primary particle size = 15 nm, MMAD = 1.1 mm). One group was exposed for 43 weeks and placed in post-exposure recovery for an additional 86 weeks in air. The other group was exposed for 86 weeks and placed in post-exposure recovery for an additional 43 weeks. The results demonstrated that no tumours were observed in control rats. In the 43-week exposure group, the lung tumour rate was reported to be 18%. In the 86-week exposure group had a lung tumour rate of 8%.