“I would think that means it meets the national guidelines — you know, for head injuries, concussions, that sort of thing,” she said. “That’s what it would mean to me.”

That assumption, made by countless parents, coaches, administrators and even doctors involved with the 4.4 million children who play tackle football, is just one of many false beliefs in the largely unmonitored world of football helmets.

Helmets both new and used are not — and have never been — formally tested against the forces believed to cause concussions. The industry, which receives no governmental or other independent oversight, requires helmets for players of all ages to withstand only the extremely high-level force that would otherwise fracture skulls.

The standard has not changed meaningfully since it was written in 1973, despite rising concussion rates in youth football and the growing awareness of how the injury can cause significant short- and long-term problems with memory, depression and other cognitive functions,especially in children.

Moreover, used helmets worn by the vast majority of young players encountered stark lapses in the industry’s few safety procedures. Some of the businesses that recondition helmets ignored testing rules, performed the tests incorrectly or returned helmets that were still in poor condition. More than 100,000 children are wearing helmets too old to provide adequate protection — and perhaps half a million more are wearing potentially unsafe helmets that require critical examination, according to interviews with experts and industry data.

Awareness of head injuries in football was heightened last weekend when helmet-first collisions caused the paralysis of a Rutgers University player, a concussion to Philadelphia Eagles receiver DeSean Jackson and injuries to three other N.F.L. players. Although some injuries are unavoidable results of football physics, helmet standards have not kept up with modern football, industry insiders said. The one helmet standard was written by the National Operating Committee on Standards for Athletic Equipment, or Nocsae, a volunteer consortium that includes, and is largely financed by, the helmet makers themselves. Nocsae accepts no role in ensuring that helmets, either new or old, meet even its limited requirement.

One frustrated vice president of Nocsae, Dr. Robert Cantu of the Boston University School of Medicine, said the organization has been “asleep at the switch” for five years. Cantu joined other prominent voices involved in youth sports concussions in calling for stronger standards.

Recent engineering advances made by Riddell, Schutt, Adams and other manufacturers have undoubtedly improved the performance of the football helmet, which from its leather roots has always symbolized football’s duality of valor and violence. But helmets communicate a level of protection that they do not provide, experts said, in part because of lax industry standards and practices.

As she looked again at the helmet of her 11-year-old son, Hunt, Ms. Sparks said: “You just trust. You care so much about your kid, and then you just trust.”

One Limited Standard

After more than 100 high school and college football players in the 1960s were killed by skull fractures and acute brain bleeding, Nocsae was formed to protect players against the extreme forces that caused those injuries. The resulting standard, phased in by all levels of football through the 1970s, requires helmets to withstand a 60-inch free fall without allowing too much force to reach the skull.

This standard has accomplished its intent: skull fractures in football have essentially disappeared, and the three or four football-related deaths each year among players under 18 are caused by hits following a concussion that has not healed (known as second-impact syndrome) rather than by a single fatal blow.

As the size and speed of players have increased since the full adoption of the Nocsae standard in 1980, concussion rates have as well. An estimated 100,000 concussions are reported each season among high school players alone, according to Nationwide Children’s Hospital in Columbus, Ohio, but many times that figure are believed to go unreported or unrecognized.

Preventing concussions — which are typically marked by confusion, disorientation, nausea and other symptoms following a blow to the head — is trickier than preventing skull fractures. The brain can crash into the inside of the skull through a wide range of forces, some arriving straight to the head and others suddenly rotating it. Scientists have yet to isolate where thresholds are in different players at different positions and at different ages.

While bicycle helmets are designed to withstand only one large impact before being replaced, football helmets can encounter potentially concussive forces hundreds of times a season. Helmets cannot get too large or heavy, so helmet designers say they face a trade-off: make helmets stiff enough to withstand high impacts and allow less violent forces to cause concussions, or more softly cushion against concussive-type forces while allowing large impacts to crack the skull.