Ingestion of milk is a primary pathway by which radioactive materials released into the atmosphere may reach the offsite population.  Milk samples are collected at areas close to the INL boundary and distant to the INL.  Samples are collected from both single-family farms and commercial dairies.  A 2-liter (0.5-gallon) sample is obtained from each location monthly, except in Idaho Falls where a sample is collected weekly. Milk from each location is analyzed for Iodine-131 (¹³¹I), and one analysis for strontium-90 and tritium at each location is performed during the year.

Milk was the first agricultural product to be monitored, beginning in at least 1957. The number of samples collected per year has been relatively constant since about 1962. Because of improvements in counting technology, the detection limit for 131I has decreased from about 15,000 pCi/L in early sampling to the current detection level of about 2 pCi/L.