Prior studies have demonstrated a seasonal pattern in hair loss, where maximal proportions of telogen hairs occur in the summer, and maximal hair shedding occurs in August and September in northern hemisphere countries. The nature of this relationship has not been understood, though temperature and daylight levels have been hypothesized to play a role. In this study, we explore the relationship among seasonality, temperature, and search queries for “hair loss” using Google Trends data across multiple countries and continents. Google Trends is an online, open-access database that has aggregated Google search data since 2004. Monthly SVI data were evaluated worldwide and from 8 countries. Of the top 15 countries by “hair loss” SVI from Jan 2004 to Oct 2016, the top 4 countries by population in the northern and southern hemispheres were selected. Multivariable regressions were conducted to examine the relationship between hair loss SVI and seasonality, as well as with temperature, hemisphere, and nominal GDP per capita. Trends in monthly “hair loss” SVI are cyclical, such that patterns of relative peaks and troughs in SVI repeat annually. Across all 8 countries analyzed in aggregate, summer and fall were associated with greater “hair loss” SVI compared to winter and spring (regression coefficient 9.26 [p<0.0001], 7.20 [p<0.0001]). Temperature did not contribute to SVI findings (p=0.34). However, countries with higher per capita GDP and located in the northern hemisphere each correlated with higher SVI (regression coefficients 0.60 [p<0.0001], 19.1 [p<0.0001]). These patterns suggest that hair loss is correlated to seasonality and that hair loss occurs most frequently in the summer and fall. Temperature did not correlate with hair loss SVI, suggesting that other factors contribute to seasonal hair loss, and that future studies exploring the effect of UV index variation on patterns of hair loss may be warranted.

Seasonality of hair loss: A digital epidemiology analysis from 2004 to 2016