This study applied Thom's Discomfort Index (DI), Kibler Discomfort Index (THI1) (Temperature-Humidity Index) and National Oceanic Atmospheric Administration (NOAA) Discomfort Index (THI2) to assess the trend of thermal discomfort in Beirut City during )1999-2021(. Temperature and Relative Humidity data for 22 years (1999-2021) from Beirut-Rafic Hariri International Airport station were analyzed to determine the discomfort period. Seasonal variations in thermal conditions were found, with increased thermal stress levels occurring mainly during Summer, while Spring and Winter showed improved thermal comfort conditions with respect to Summer. December, January, and February were comparatively comfortable (DI < 21). Under 50% of the population experienced discomfort (21 < DI < 24) during March and November. From May to September, discomfort values varied between 25-27, which indicates that over 50% population in Beirut suffered from discomfort during these months. Percentage frequencies of the monthly (DI) values from January to December in Beirut from the years 1999 to 2021 were reported. Monthly variations of (THI1) and (THI2) showed no extreme discomfort conditions in Beirut. Data on air temperatures and (RH) in Beirut were investigated to study local climate change and understand potential impacts of urbanization on Beirut's climate because of industrialization and a significant migration from rural areas. An upward trend in annual average air temperatures and downward trend in the annual average (RH) of Beirut, the most highly populated and industrialized city of Lebanon, were found. The results of this study are crucial for tracking human thermal comfort /discomfort in Beirut's educational and other places.