Alcohol fireplaces produce primarily carbon dioxide and water vapor; however, they may also produce toxic substances. Some research concludes alcohol fireplaces without chimneys are unsafe for indoor use, especially if there is not an adequate source of ventilation.
All alcohol fireplaces consume oxygen in the air, so they must not be used unless this is both a source of make-up fresh air.
And if they do not have a chimney, there must also be means to exhaust air from the room (as the air may contain carbon dioxide and pollutants from the combustion).
If you have sufficient fresh air from ventilation, these fireplaces may be an option is used carefully. An addition benefit in some climates where winter humidity falls below healthy levels is that the combustion of alcohol produces water vapor and adds humidity.