Temperature

Comfortable bathing sits roughly between 38 and 40 °C. Above about 45 °C the water is unpleasant within seconds; near boiling it causes full-thickness burns almost instantly. Geothermal water does not steam predictably, so appearance is a poor guide.

Temperature also stratifies. A pool can be pleasant at the surface and dangerous at the inflow, and the inflow is often at the bottom where you cannot see it. Enter slowly, never jump, and keep a hand on a route out.

Ground

In high-temperature areas the surface can be a thin crust over hot mud. Marked paths exist because the ground beside them has already failed. Off-path walking in these fields is the single most common cause of severe burns.

Around unmanaged pools, look for undercut banks and loose scree above the water. Both fail more easily when wet.

Weather and the walk out

The hazard is usually not the bathe, it is being wet, cold and an hour from the vehicle in changing weather. Icelandic conditions can move from calm to driving sleet within an hour, and rivers rise fast in rain or a warm spell.

Check the national forecast and the road status before setting out, and treat any river crossing on foot or by vehicle as a decision that can be made twice — once out, once back, in worse conditions.

• Carry dry insulating layers in a waterproof bag, not just a towel.
• Plan the exit for the weather at the end of the day, not the start.
• Turn back at the crossing you would not want to repeat.

Water quality

Warm standing water grows things. Untreated pools that are lightly flushed can carry bacteria, and warm freshwater anywhere in the world carries a small risk to the nasal passages, so keep your head above water in unmanaged pools.

Do not bathe with open wounds, do not use soap or shampoo in natural water, and follow the local rule of showering thoroughly before entering any public pool.