Iron oxide is one of 16 chemical compounds composed of iron and oxygen, or iron, oxygen and hydrogen. Many of these compounds occur naturally; rust, for example, is a result of iron reacting to water, which is made up of hydrogen and oxygen molecules. Others are made deliberately for industrial purposes. Penny coins, for example, are made from metal pigmented with iron oxide compound. Iron oxide is also used for its magnetic properties. Although generally considered nontoxic, iron oxide compounds have been investigated for possible harmful effects to humans.
There are various medical uses for iron and iron oxide, as both are naturally occurring and essential to the human body. However, recent research by UC San Diego has suggested that iron oxides used commonly in medical treatments might cause toxicity in some cell types when paired with specially developed coatings intended to bind the iron oxide particles to particular cell types, like cancerous cells. In a paper published by Biomaterials, senior author Sungho Jin suggested that while neither the iron oxide or the coatings were individually harmful, in combination they caused unexpected reactions in the cells they bind to.
Large amounts of iron oxide can cause serious environmental issues. In mining states like Pennsylvania, iron mines flooded with water spill gallons of iron oxide into ponds, rivers and lakes; this can poison fish and pollute the environment. The iron molecules react with oxygen to create a solid iron oxide known as "yellow boy," which sinks to the bottom of rivers and waterways to pollute them. "Yellow boy" chokes out aquatic life and kills fish and plants.
In Australia, storms whipping red iron oxide dust into the atmosphere of city centers has revealed that iron oxide can cause breathing problems and exacerbate asthma, allergies and sinus infections. The iron oxide itself isn't so dangerous as the plant pollen and material it attracts to it; this material can cause serious and potentially life-threatening allergic reactions.
Iron oxide is frequently used to make magnets, and these have been linked to a variety of health problems. The most common danger of small iron oxide magnets is that they might get swallowed. Two magnets inside the digestive tract can bind together between intestinal walls, creating a blockage that can have serious or even deadly medical repercussions. This is especially dangerous with infants and toddlers, who can easily swallow small magnets.