Which of the following is considered a common contaminant of lubricants?

Water is widely recognized as a common contaminant of lubricants due to its propensity to enter lubricating systems through various means such as condensation, leakage, or improper sealing. The presence of water in lubricants can severely compromise their effectiveness by promoting oxidation, reducing lubrication properties, and facilitating the growth of microorganisms that can cause further damage to equipment.

Water can lead to increased wear and tear on mechanical components, create corrosion, and form emulsions that alter the lubricant's characteristics. This makes it crucial for reliability engineers to monitor and control the levels of water contamination to ensure the longevity and performance of machinery.

Other options, while they can impact lubricant performance, do not qualify as direct contaminants in the same way that water does. Heat, for instance, is a byproduct of friction and can degrade the lubricant, but it doesn't enter the lubricant as a contaminant. Dust may enter lubricating systems leading to wear, but it is not as universally recognized or prevalent as water. Steam, similar to water, may affect lubricants but is more specific to situations where water vapor condenses. Therefore, water stands out as the primary and most common contaminant affecting lubricants.