Background: Air pollution has been considered a potent environmental risk factor for neuropathology through neuroinflammation and oxidative stress, which might also cause brain tumour formation. However, epidemiological evidence on the association between air pollution and brain tumours in humans is sparse, with no data on exposure to particles. In this study we aim to examine associations between long-term exposure to ambient air pollution and risk for development of brain tumours. Methods: We used the Danish Nurse Cohort with 28,731 female nurses (age ≥ 44 years) recruited in 1993 or 1999 when self-reported information on lifestyle was collected. We obtained data on the incidence of brain tumours until 2013 from the Danish Cancer Register, and estimated annual mean concentrations of particulate matter with diameter < 2.5 μm (PM2.5), particulate matter with diameter <10 μm (PM10), nitrogen oxides (NOx) and nitrogen dioxide (NO2) at the residence since 1990 using an atmospheric integrated chemistry-transport models system, and examined the association between the 3-year running mean of pollutants and brain tumour incidence using time-varying Cox regression, separately for total brain tumours, and for tumour subtypes by location (brain or meninges), and by malignancy (malignant or benign), and estimated hazard ratios and 95% confidence intervals per increase in interquartile range of exposure. Results: Of 25,143 tumour-free nurses at recruitment, 121 developed brain cancer during 15.7 years of follow-up. We found a weak positive association between total brain tumours and PM2.5 (1.06; 0.80-1.40 per 3.37 μg/m3), NO2 (1.09; 0.91-1.29) per 7.5 μg/m3, and NOx (1.02; 0.93-1.12 per 10.22 μg/m3), and none with PM10 (0.93; 0.70-1.23 per 3.31 μg/m3). Associations with PM2.5 and NO2 were stronger for tumours located in meninges than in brain, and for benign than for malignant tumours. Finally, association of total brain tumours with PM2.5 was modified by BMI, and was statistically significantly enhanced in obese women (2.03; 1.35-3.05). Conclusion: We found weak evidence for association between risk of brain tumours and long-term exposure to air pollution in women older than 44 years. However, we present novel results that obese women may be susceptible, as well as a positive tendency towards elevated risk for meninges and benign tumours, which require further investigation.