Threshold Odor Number Calculator

A TON of 3 means the sample had to be diluted to one-third its original concentration before the odor became barely detectable. This is the EPA secondary drinking water standard limit — water at or above TON 3 typically generates consumer complaints about taste and smell.

A trained sensory panel prepares a series of dilutions using measured volumes of sample water and odor-free water. Each flask is heated to 60 °C and compared to an odor-free blank. The threshold dilution is the lowest one at which the panel consistently detects an odor difference from the blank. Standard Methods 2150B specifies the procedure.

Most drinking water standards set a maximum TON of 3. Water with TON above 3 typically triggers consumer complaints about taste and smell. The EPA secondary standard is non-enforceable but widely used as a quality benchmark.

Common causes include algal blooms (geosmin and 2-MIB from cyanobacteria), hydrogen sulfide from anaerobic conditions, chlorine or chloramine disinfection byproducts, and industrial contamination. Seasonal algal blooms are the most frequent cause of taste-and-odor episodes in surface water supplies.

Heating to 60 °C increases the volatility of odor-causing compounds, making them easier for the sensory panel to detect. This gives a more sensitive and reproducible result than testing at room temperature, where some compounds might be present but not volatile enough to smell.

Yes, though wastewater applications often use related tests like the D/T (dilution-to-threshold) ratio from dynamic olfactometry. The standard TON test works for liquid samples, while olfactometry measures airborne odors from treatment facilities. Both quantify odor intensity as a dilution ratio.