Ladies and gents, you’re over worrying about humidity when you should be concentrating on locking down your temperature in order to get your lash glue operating at its best.
If your trainer frightens you to death about humidity, take it from the glue dude – they’re wrong and glue chemistry says so!
Most glues don’t work well below 20C (70F). If you lash at or below this temperature and you constantly have issues with your glues/retention, the problem is incorrect temperature.
The ideal temperature for pretty much all glues on the market is 21-26C (70-78F, the higher the temperature the faster the glue will set), and most if not all will work between 30-65% humidity. This is why you don’t need to worry about humidity because most of time, you will be inside this wide range. You will only need to think about humidity if your room is outside of 30-65%.
For fast glues (sub 2 secs), it’s best to stick to the lower half of the ideal temperature range (21-23C). If you want your intermediate glues (2-4 sec) to set faster time, set your temperature to the top half of the ideal range (24-26C).
Aircon/heater plays a vital role in your environment. If you have them on, make sure you’re not in the direct path of the vent, as the air blowing out is extremely dry and cold/hot (ac/heater). Your hygrometer measures the ambient conditions and the difference between what’s coming out of the vent (and hitting you) and the ambient can be huge… Hence looking at your hygrometer, you may think that your room is ideal, but when in fact the surrounding air (around the clients head) could be very different if you’re right in the path of the vent. (See picture above.)
Over the past couple of years, I’ve noticed that this same “glue” issue always crop up around summer and winter… Why? Because during November – February, Europe/US/Canada (northern hemisphere) are going into winter (requiring the heater to be on), and summer for Australia/NZ (southern hemisphere) – requiring ac to be blasting full. This is reversed around June-August.
And how’s this for a bonus for reading this far – the most efficient temperature to keep your room at is 22-24°C (~72°F) – ie this is the temperature you set your heating/cooling unit at. Perfect 
Double bonus – all you need then is just a humidifier (instead of a dehumidifier or both) because both, the ac and heater, will remove humidity from the room. Only use the humidifier when it drops below 30/35% (or when your glue can’t cope). Ideal humidity is 30-65% for most adhesives.
Set and forget at 23C/74F!
Here are a few more workarounds when you need to use the aircon or heater.
