Dehydration and machine operation Drink to thirst is a principle. You cannot set up hard and fast rules because every person is different, as is their working environments. When you say 'older people" it might be helpful if you referenced it as those broadly over the age of 65 - where it is a problem for those who live independently and living in a "heat wave" where their sensation can't be relied upon to signal the need to drink. Its a big problem in retirement villages and rest homes. Not one, I would have thought for your average forklift driver.
But back on topic "Other authors have found that the thirst sensation does not begin until about 1–2% of body weight or 2% of total body water has been lost."
As another broad principle you can estimate fluid loss through sweat at around 1.0 - 1.5 litre an hour under quite some exertion. Heres an exercise you can try at home. Jump on the scales and go for a walk or a run for an hour. Jump back on the scales and see how much weight you have lost. Most of it will be through sweat with some of it being loss of stored energy and respiration. (My last tested personal benchmark is 1.2kg an hour running in 25 degrees at 80+% max heart rate. Drink to thirst kicks in at about 50 minutes. C'mon forum members share your results!). Once you have tried this, then try it on your average forklift operator or driver. My money is on a result that won't come close to this weight loss.
Humour me - and lets say they loose 1/2 kg an hour in tough, but not extreme temperature conditions. That gives them 2 hours work with no detrimental effects. (In Christchurch here we do get such conditions - rarely in summer when the hot nor wester is coming through. Today its a balmy 14 degrees with gentle southery)
You want my citation - bare with me. "In this regard, there is disagreement in the literature about acceptable sweat rates for industrial workers. While sweat rates of 1.5–2.5 l/h have been shown over short periods (with peaks of 3 l/h), acceptable figures for a working shift are generally considered to be lower. ISO 793313 and Belding and Hatch advocate a limit of 1.04 and 1.0 l/h respectively for acclimatised persons, although ISO 988615 curiously states that “There is no limit applicable concerning the maximum sweat rate: the values ... adopted in ISO 7933 ... must be considered not as maximum values but rather as minimal values that can be exceeded by most subjects in good physical conditions”. Nunneley reports that humans can sweat indefinitely at rates of 1.5–2.0 l/h, while McArdle and colleagues recommended a limit of 4.5 l over four hours."
Lets agree to disagree and settle on an acceptable sweat rate of 1 l/h. Now that's going to be for an acclimatised worker - which is what our forklift operators will be (the exceptional one off hot days aside). That gives them an acceptable work period of 2 hours. I call that "quite some time" and we haven't even touched on "detrimental effects" at this point.
This paper here is interesting reading:
https://oem.bmj.com/content/60/2/90#ref-21 . To save some effort it does involve research involving extreme working conditions (>28 degrees) and extreme work (underground miners). Your average Forklift Operator isn't going to come close to the condition extremes these miners work under.
Boils down to: under usual work conditions a cuppa at smoko will do the trick.