Complex sentence: Construction workers' working conditions in this tunnel have been reported as particularly difficult: with the temperature in the tunnel reaching 43 degrees Centigrade and relative humidity, 98%, it was nicknamed the "sauna tunnel" by the workers.
Split into simpler sentences: Construction workers' working conditions in this tunnel have been reported as particularly difficult, with the temperature reaching 43 degrees Centigrade and relative humidity, 98%. It was nicknamed the "sauna tunnel" by the workers.