Coaches benched Steelers safety Ryan Clark this week due to fear of the complications that he faced last time he played in Denver, which included life-threatening illness.

Four years ago, the combination of Clark’s sickle cell trait and Denver’s high altitude caused organ damage, and surgeons had to remove his spleen and gallbladder.

One in 12 African-Americans has sickle cell trait. However, having the trait rarely causes such extreme illness.

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Sickle cell trait is different from sickle cell disease. Usually people with the trait have no symptoms and live normal lives without worry. Having sickle cell trait means a person has only one copy of the sickle cell gene. Those with the disease inherit two copies — one from each parent.

With sickle cell disease, red blood cells become sticky and look like a C-shaped farm tool called a “sickle” during times of bodily stress. Weather changes, illness, dehydration, low oxygen and high altitude are all triggers.

Sickled cells block arteries, causing severe pain throughout the body. Because sickled red cells die quicker, those with the disease have anemia. Sickle cell disease also raises the risk of infection, stroke and vision loss.

Since Denver is a mile above sea level, the low oxygen caused Clark’s cells to sickle, damaging organs, including his spleen.

This level of severity is unusual since he only has the trait, making the case that sickle cell trait may not be as benign as once thought. Sickle cell trait has also been highlighted in the media recently for its link to sudden cardiac death.

The biological mechanism is unclear. But, scientists know that rigorous athletic activity in athletes with sickle cell trait in intense heat, while dehydrated or without appropriate conditioning raises the risk of sudden death.

A 1970s study of military recruits also found an association between sudden, unexplained deaths and sickle cell trait.

By 2000, nine deaths of NCAA athletes had been attributed to the trait. And, experts at the University of Wisconsin predict that without safer training measures for athletes with sickle cell trait, seven more athletes would die in the next 10 years.

In response to a lawsuit by the parents of a deceased college freshman with sickle cell trait, the NCAA passed a regulation mandating that Division I athletes be tested for it unless they sign a waiver. It isn’t meant to keep them from participating, but rather to tailor their conditioning.

People of African descent and other cultures where malaria is prevalent carry the trait more often. The parasitic disease infects red blood cells and replicates within them. In patients with sickle cell trait or disease, the red cells sickle and die from the stress thus killing the malaria before it spreads.

Africans without the sickle cell gene often died from malaria, leaving behind a population of Africans who continued pass on the sickle cell gene.

If both parents have sickle cell trait, there is a one in four chance that their child will have the full sickle cell disease, and a 50 percent chance that their child will have the trait. If someone with sickle cell disease has a child, all of their children will have sickle cell trait, even if the other parent does not.

All 50 states and the District of Columbia now screen newborns for sickle cell disease and trait. However, many parents either don’t remember or aren’t appropriately counseled on the results.

Clark says he knew he had it, but underestimated his risk.