Special type of SMD resistors from Bourns that are resistant to most environmental pollution. Link here (PDF)
In recent years, the long term reliability of film chip resistors in a high sulfur environment plays an increasingly important role in a wide range of market segments such as industrial electronics, IT systems and telecommunications. Sulfurous environments have a negative impact on film resistors by reacting with elements, in particular silver. Sulfide growth can cause resistors to go open circuit.
The critical environmental factors of sulfur pollution are:
• Where oils, lubricants or fossil fuels are used
• Rubber seals, gaskets, grommets or molded coatings of affected material
• Air polluted industrial area or areas containing biological or volcanic gasses