I usually replaced the HSI when cleaning the flame rod, especially if it was 3 to 5 years old. The newer ones (nitrile) would last longer than the nitrides.
You're right about krap in the air. Went into a beauty salon for a gas leak. Furnace was less than 7 years old. Hair spray and hair dies ate that 80% right up. She changed to 90+, with sealed combustion chamber, getting intake air from outside. She had went with 80% furnaces because her furnace guy "recommended" that they were more reliable. I changed her mtr out probably 5 years later, 90+ was looking good in the burner area because it wasn't sucking all those chemicals in anymore.
You're right about krap in the air. Went into a beauty salon for a gas leak. Furnace was less than 7 years old. Hair spray and hair dies ate that 80% right up. She changed to 90+, with sealed combustion chamber, getting intake air from outside. She had went with 80% furnaces because her furnace guy "recommended" that they were more reliable. I changed her mtr out probably 5 years later, 90+ was looking good in the burner area because it wasn't sucking all those chemicals in anymore.