Ehrström was appointed to the position of municipal veterinarian in Helsinki in 1918. His task was to organise milk inspection in the city to protect people from the dangers of milk. The city had launched quality inspections of sold milk as early as the late 19th century. Ehrström extended the inspections to cover all stages of milk production from the farm via the dairy to the shop. He was convinced that the hygiene of milk depended on the production conditions. Cows had to be healthy and their shelters clean.
Milk producers were initially suspicious of Ehrström making inspection calls on their farms. Nevertheless, he created an inspection system based on education, encouragement and incentives. The system proved effective. Diseases spreading through milk became rarer and the quality of milk improved significantly. By the mid-1930s, almost all Finnish cities had organised milk inspection following Helsinki’s example.
Ehrström was determined, persistent and maintained extensive networks. He participated in the preparation of the 1947 milk inspection act, which required population centres of more than 4,000 inhabitants to inspect all the milk sold there. In 1974 all milk produced had to be inspected. Current EU legislation related to milk production is largely based on the same principles that Walter Ehrström advocated as early as the 1920s.