The 6-nines rule in incinerators and boilers refers to which level of emission reduction?

• A. 6-nines rule
• B. 3-nines rule
• C. 2-nines rule
• D. 5-nines rule

Answer: (A)

In this context, emission performance is described using the number of nines, a shorthand that communicates how tightly pollutants must be controlled. Each additional nine tightens the target dramatically. A three-nines rule corresponds to about a 99.9% reduction, a two-nines rule to about 99%, and a five-nines rule to about 99.999% reduction. The six-nines rule goes further: it targets a 99.9999% reduction, meaning only about one part per million of the pollutant could remain in the exhaust. This is why the six-nines rule represents the strictest level among the options for incinerators and boilers.