The best approach is to stick to natural fibre (cotton) and have a
layered approach to clothing. Deserts can be hot during the day and
cold at night, so thin short sleeve cotton overalls during the day,
hard hat with neck protection from the sun. Body warmer for the the
cold nights.
Gloves are a must, metal temperatures in the tropical regions can
reach 80 degrees centigrade in direct sunlight. Strong ankle length
laced boots to provide good grip on rough terrain and to give ankle
support and minimise the risk of a twisted ankle (slip on rigger
type boots do not provide this support).
There is a huge list of standards for the thermal environemnt
including clothing on HSE's website at
http://www.hse.gov.uk/temperature/furtherinfo.htm
These appear revelant to your needs:
BS 7963 Ergonomics of the thermal environment – Guide to the
assessment of heat strain in workers wearing personal protective
equipment
BS EN 563 Safety of machinery – Temperatures of touchable surfaces
– Ergonomics data to establish temperature limit values for hot
surfaces
BS ISO 9920 Ergonomics of the thermal environment – Estimation of
the thermal insulation and evaporative resistance of a clothing
ensemble
Unfortunately there is no straightforward answer to your question
because temperature can vary greatly in any part of the world and
heat stress is also dependant of the level of physical activity
being undertaken, the fitness of an individual so a tiered approach
to risk control should be used:
- Selection of appropriate clothing
- Minimising work in direct sunlight
- Minimising work around midday
- Frequent breaks in air conditioned shelters
- Rehydration, cold water readily available
- Training - awareness of symptoms of heat stress
- Medical support in case of heat exhaustion
