Simply put: diesel emissions. In underground metal mining, the vast majority of machines run on diesel engines. These are known to have low energy efficiency that leads to high fossil fuel consumption, high heat emissions and greenhouse gases. According to mining regulations, an enormous amount of fresh air is needed to dilute emissions.
This leads to high ventilation expenses associated with rising electrical demands, which severely affects mining costs. And to make matters worse, evidence shows that exposure to heat emissions and greenhouse gases is linked to an increased risk of developing lung cancer (IARC 2012), thereby putting miners in health-compromising situations.