Just two hours of vigorous work in the summer sun without drinking fluids could be enough to blunt concentration, say scientists who found that cognitive functions often deteriorate as the body gets dehydrated.
Researchers at the Georgia Institute of Technology in the US analysed data from multiple research papers on dehydration and cognitive ability.
The data pointed to functions like attention, coordination and complex problem solving suffering the most, and activities like reacting quickly when prompted not diminishing much.
"The simplest reaction time tasks were least impacted, even as dehydration got worse, but tasks that require attention were quite impacted," said Mindy Millard-Stafford, a professor at Georgia Tech.
As the bodies of test subjects in various studies lost water, the majority of participants increasingly made errors during attention-related tasks that were mostly repetitive and unexciting, such as punching a button in varying patterns for quite a few minutes.
There are situations in life that challenge attentiveness in a similar manner, and when it lapses, snafus can happen.
"Maintaining focus in a long meeting, driving a car, a monotonous job in a hot factory that requires you to stay alert are some of them," said Millard-Stafford, a principal investigator of the study published in the journal Medicine & Science in Sports & Exercise.
"Higher-order functions like doing math or applying logic also dropped off," said Millard-Stafford.
The researchers have been concerned that dehydration could raise the risk of an accident, particularly in scenarios that combine heavy sweating and dangerous machinery or military hardware.
There's no hard and fast rule about when exactly such lapses can pop up, but the researchers examined studies with one to six percent loss of body mass due to dehydration and found more severe impairments started at two percent.
That level has been a significant benchmark in related studies.
"There's already a lot of quantitative documentation that if you lose 2 percent in water it affects physical abilities like muscle endurance or sports tasks and your ability to regulate your body temperature," said Millard-Stafford.
"We wanted to see if that was similar for cognitive function," she said.
The researchers looked at 6,591 relevant studies for their comparison, then narrowed them down to 33 papers with scientific criteria and data comparable enough to do metadata analysis.
They focused on acute dehydration, which anyone could experience during exertion, heat and/or not drinking as opposed to chronic dehydration, which can be caused by a disease or disorder.
"If you weigh 200 pounds and you go work out for a few of hours, you drop four pounds, and that's 2 percent body mass," Millard-Stafford said.
"With an hour of moderately intense activity, with a temperature in the mid-80s, and moderate humidity, it's not uncommon to lose a little over 2 pounds of water," she said.
"If you do 12-hour fluid restriction, nothing by mouth, for medical tests, you'll go down about 1.5 percent. Twenty-four hours fluid restriction takes most people about 3 percent down," she said.
Older people can dry out more easily because they often lose their sensation of thirst and also, their kidneys are less able to concentrate urine, which makes them retain less fluid.
People with high body fat content also have lower relative water reserves than lean folks.