(a) placing poultry eggs into an egg storage room at the hatchery, wherein the egg storage room is maintained at a first relative humidity;

(b) storing at least a portion of the poultry eggs in the egg storage room for a storage period, wherein the temperature of the egg storage room during the storage period is below physiological zero;

(c) transferring at least a portion of the poultry eggs from the egg storage room to an incubation room;

(d) incubating the poultry eggs in an incubator for an incubation period, wherein the incubating is carried out at a temperature of at least 37° C. and a second relative humidity lower than the first relative humidity in the egg storage room, wherein the incubator is located in a protected area of the incubation room, wherein said protected area reduces an exchange of air with an environment outside the incubation room to prevent dilution of the DHP;

(e) during the incubating, continuously treating the poultry eggs with DHP at a concentration in the range of from 0.001 ppm to 10 ppm to provide treated eggs; and

(f) hatching at least a portion of the treated eggs to provide a plurality of chicks.