Humans occupied a wide geographic range, living under all kinds of environmental conditions. It is important to mention that the environment not only comprises of external factors like temperature but also internal factors. There are four main ways in which humans respond: cultural practices and technology, acclimatization, developmental adjustment, genetic adaptation. Acclimatization: can be defined as physiological adjustment of individual organisms to different conditions (like temp, altitude, photoperiod) Is a reversible process, entails no genetic change: length of time in which these reversible changes occur can be quite variable. Sometimes they occur very quickly (short-term acclimatization), sometimes they can take much longer, even ( long -term acclimatization: e. g. Increase in respiration rate, heart rate, and red blood cell production when moving to high-altitude places: tanning response under increased uv radiation. Immediate pigment (photo oxidized) ipd(within 1-2 hours of exposure, fades away during the first 24 hours.