In the second trial (Figure 1b), the 2 experimental groups (A and E) received 37% formaldehyde through CRI during hatching and group C had a significantly superior inhibitory effect on aerosol bacterial counts 24 h after transfer than the negative control group (D) (P < 0.05). The 2 experimental groups (A and E) had a significantly superior inhibitory effect on aerosol bacterial counts 4 h before hatching than group C. There was no significant difference in the level of bacterial contamination between experimental group A, which received 226.8 g of 37% formaldehyde through CRI at the time of transfer, an additional dosage every 12 h, and a final dose at 12 h before chick removal, and group E, which received 226.8 g of 37% formaldehyde through CRI at the time of transfer, an additional dosage every 12 h, and a final dose of 283.5 g at 12 h before chick removal.