Artificial light at night (ALAN) for elongating photophase is a new source of pollution. We examined the association between measured ALAN levels and breast cancer (BC) standard morbidity ratio (SMR) at a statistical area (SA) level in an urban environment. Sample size consisted of 266 new BC cases ages 35-74. Light measurements (lux) were performed in 11 SAs. A new calculated variable of morbidity per SA size (SMR/km) was correlated with the light variables per road length, using Pearson correlations (P < .05, 1-tailed). Looking for a light threshold, we correlated percentage of light points above SA light intensity median with SMR/km. SMR/kmwas significantly and positively strongly correlated with mean, median, and standard-deviation (SD) light intensity per road length (r = .79, P < .01, R= .63; r = .77, P < .01, R= .59; and r = .79, P < .01, R= .63). Light threshold results demonstrate a marginally significant positive moderate correlation between percentage of points above 16.3 lux and SMR/km(r = .48, P < .07; R= .23). In situ results support the hypothesis that outdoor ALAN illumination is associated with a higher BC-SMR in a specific area and age group. Moreover, we suggest an outdoor light threshold of approximately 16 lux as the minimal intensity to affect melatonin levels and BC morbidity. To the best of our knowledge, our attempt is the first to use this method and show such association between streetlight intensity and BC morbidity and therefore should be further developed. 35-74 35-74 35-74 35-74 2 2 2 2 2 2 2 2
