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The following signs indicate that a coolant is no longer safe to use and might be hazardous to workers health:

• Foul smell. Uncontrolled microbial growth causes metal working fluid to stink. It is important to address the cause of rancid smells, as the fine spray that is created during machining contains live microbes which can cause harmful health effects (dermatitis) in the employees who are exposed to this coolant mist.  Foul smelling fluid is likely caused by microbial growth and needs to be analyzed and treated with biocide. Coolant that cannot be saved should be discarded appropriately. Before adding fresh coolant, the sump needs to be thoroughly cleaned.

• Abnormal fluid appearance. Check the color and appearance of the fluid regularly. Usually, synthetic fluids are clear, often the semi-synthetics appear transparent to milky, and soluble oil typically has a milky appearance with no oil layer. Bacteria often cause coolants to turn grey or black. Tramp oil may cause yellowing and browning of coolants. Colorless coolant is a sign of depleted coolant.  

•  Employees suffer from respiratory irritation. Exposure to coolant mist that is contaminated by bacteria and fungal spores can lead to irritation and tightness in the chest.

• Employees suffer from dermatitis skin symptoms from working with metalworking fluid, a persistent itchy rash that lasts six months or more.