Regulation on disinfection of hatching eggs (which should be called “disinfection of egg SHELLS in reality; once microbes got inside the eggs through its thousands of pores its too late) is regulated differently country by country.
Formaldehyde is carcenogenic and doesn’t kill spores of Aspergillus fumigatis.
In the past, we’ve done research on 9.7 million eggs and chicks with Virocid, but, again, not every country allows hatching egg disinfection.
Before setting is the best time, but eggs shold be allowed to dry before they enter the setter.
Also on the farm, a first disinfection is advisable (when allowed). Thus no contaminated eggs enter the egg reception room.