You can use both of them to sterilize things, and they’re both potentially dangerous. What are bleach and formaldehyde, and how are they different?
History and production
Humans have been “bleaching” things for a long time; in the past, we used to just leave things out in the sunlight to whiten them, and we’ve used other chemicals over time which aren’t related to what we think of as “bleach” today. Modern bleach was discovered in Sweden in the 18th century. Most bleach now uses chlorine to sterilize and whiten, but in its pure form that’s a gas, so we tend to use it in other forms like sodium hypochlorite (NaOCl). A few other chemicals, like hydrogen peroxide, may also be used for the same kinds of purposes, and some of them are “color-safe”.
Formaldehyde is a hydrocarbon (CH2O), which was discovered in Russia in 1859 and is usually produced today by oxidizing methanol. It does occur naturally in the atmosphere and has also been discovered in deep space. It even forms in biological processes in small amounts and can be found in the human bloodstream.
Bleach is common in many households and is used to lighten or whiten things and remove stains. For example, it’s common when washing white clothing or other fabrics to include some kind of bleach to help keep everything looking bright white. It’s used for the same reason in industry, to remove the color from products like wood and make them whiter. The chemicals in bleach break down color molecules in dyes so that they no longer look colorful. Other than that, we mostly use bleach to clean and disinfect things. Commercial use for bleach would be places like swimming pools, to keep the water and other surfaces free from microbes. In the house, we often use a bleach solution to wipe down surfaces, especially in kitchens and bathrooms, to keep them germ-free.
Formaldehyde can also be used to clean and disinfect, but that’s not what we usually think of. It’s used heavily in the industry to make other chemical products, and also in resins to treat woods like particleboard. The other major commercial use for formaldehyde is to preserve dead tissue. It’s used for this purpose by both biologists and funeral homes. Other than that, it’s sometimes used for its disinfectant properties in cosmetics, animal food, and a few more places, but it’s not too commonly used in homes.
Health and environmental concerns
As you might guess, anything which kills germs isn’t good for human health either. Pure chlorine gas is very toxic and care must be taken not to mix substances containing bleach with products that have ammonia, as this may generate pure chlorine. This gas causes chemical burns both to the skin and internally if breathed in. However, bleach is not usually considered an environmental hazard in normal quantities, because it quickly reacts with other chemicals to become inert. In larger industrial quantities, chlorine is a source of concern and can create pollutants such as dioxin.
Formaldehyde has been deemed to be a source of cancer, and humans should avoid breathing the fumes whenever possible. It tends to cause light respiratory allergies and irritation in normal quantities, but people who work in close contact with sources of formaldehyde — funeral homes with embalming fluid, chemical plants which produce resin, or construction companies working with particle board are at higher risk — should always wear protective gear and limit exposure.