Agnihotra or Hawan are traditional Bhartiye cultural ritual , where the cow ghee (or milk) , or mixing it with material (known as hawan samagri) and put it into the fire.
For more scientific information , see below link
Is There Any Scientific Basis of Hawan to be used in Epilepsy-Prevention/Cure ?
Fatty Substances Combustion:
Cow Ghee be an fatty substance that helps in rapid combustion that keeps the fire alight. All fatty substances used are combinations of fatty acids, which volatilise easily. The combustion of glycerol portion gives acetone bodies, pyruvic aldehyde and glyoxal etc. The hydrocarbons produced in the reactions again undergo slow combustion and as a result methyl and ethyl alcohols, formaldehyde, acetaldehyde, formic acid and acetic acids are resulted.
All the volatile substances diffused in the surrounding atmosphere undergo the photochemical reactions in the sunlight that is why it is recommended that Yagya to be performed in the presence of sunlight. These changes occur in the ultra-violet and other short wavelength regions. The products of fumigation thus go photochemical decomposition, oxidation and reduction. To some extent even CO2 is also reduced to formaldehyde as follows:
CO2 + H2O + 112,000 cal. HCHO + O2
From environmental angle, reduction of CO2 produced in Yagya as above and liberation of oxygen cannot be overemphasized.