ALS (amyotrophic lateral sclerosis) has no single cause. In most cases, the disease results from a combination of genetic vulnerability and environmental exposures that together trigger the progressive death of motor neurons. About 90 to 95 percent of cases appear with no family history, while 5 to 10 percent run in families with identifiable genetic mutations. Even in sporadic cases, the underlying biology involves the same destructive chain of events: proteins misfold and clump inside nerve cells, chemical signaling goes haywire, and the cell’s energy-producing machinery breaks down.
Familial vs. Sporadic ALS
The traditional split is that roughly 5 to 10 percent of ALS cases are familial, meaning the person has a known relative with the disease, while the remaining 90 to 95 percent are sporadic. A 2023 meta-analysis pooling 165 studies found the overall familial proportion was about 8 percent, though more recent individual studies have reported figures as high as 17 percent. The line between familial and sporadic is blurring as genetic testing improves. Some people classified as sporadic turn out to carry the same mutations found in familial cases, suggesting genetics plays a broader role than the simple family-history question captures.
A notable difference between the two groups shows up in who gets the disease. Overall, ALS is about 1.25 times more common in men than women. But that gap is almost entirely driven by sporadic cases, where the male-to-female ratio is 1.29. In familial ALS, the ratio is essentially equal at 1.05, pointing to the influence of non-genetic, possibly hormonal or occupational factors in the sporadic form.
Genetic Mutations Linked to ALS
Researchers have identified more than 30 genes associated with ALS, but four account for the majority of known familial cases. The most common is a repeat expansion in the C9orf72 gene, which produces abnormal stretches of repetitive DNA. This single mutation is responsible for the largest share of familial ALS cases in people of European descent and also appears in some sporadic cases. Mutations in the SOD1 gene, the first ALS gene discovered in 1993, account for another significant portion. Two additional genes, TARDBP and FUS, are less common but well established as contributors.
What these genes have in common is that they disrupt the normal machinery of motor neurons. C9orf72 mutations, for instance, compromise the cell’s energy-producing structures and increase the kind of molecular damage caused by unstable oxygen molecules. SOD1 mutations impair a protein whose job is specifically to neutralize those dangerous molecules, leaving the cell unable to defend itself.
How Motor Neurons Break Down
Regardless of whether ALS begins with a genetic mutation or an environmental trigger, the same cascade of destruction unfolds inside motor neurons. One of the most consistent findings is the buildup of a misfolded protein called TDP-43. In healthy cells, TDP-43 sits in the nucleus and helps manage RNA, the molecular instructions cells use to build proteins. In ALS, TDP-43 gets displaced from the nucleus into the surrounding cell fluid, where it clumps into insoluble aggregates. This creates a double problem: the nucleus loses a protein it needs to function properly, and the growing clumps in the cell’s interior become toxic, pulling in more normal TDP-43 and accelerating the damage.
Another major driver is a process called excitotoxicity. Motor neurons communicate using a chemical messenger called glutamate. Normally, glutamate fires briefly across the gap between neurons and is quickly cleaned up by surrounding support cells. In ALS, glutamate accumulates in that gap because the cleanup system fails. The receiving neuron gets overstimulated, flooding it with calcium. That calcium overload damages the cell’s mitochondria, its power generators, which then start producing harmful reactive oxygen molecules instead of energy. Those molecules damage DNA, proteins, and cell membranes, pushing the neuron toward death.
This feeds into a vicious cycle. Excess calcium also activates enzymes that chop TDP-43 into fragments, worsening the protein clumping problem. Meanwhile, damaged mitochondria become even less capable of buffering calcium, making the neuron more vulnerable to the next wave of glutamate signaling. The entire process is self-reinforcing, which helps explain why ALS progresses relentlessly once symptoms appear.
Oxidative Stress and Mitochondrial Failure
Mitochondria are the primary energy source for motor neurons, and they are also the main site where reactive oxygen species (free radicals) are produced as a byproduct of energy generation. In ALS, this balance tips sharply toward destruction. Studies of cells from sporadic ALS patients show decreased activity of the enzymes that generate energy, lower overall energy output, reduced ability to neutralize free radicals, and a drop in the electrical charge that mitochondria need to function. In essence, the power plants of the cell are simultaneously producing less energy and more toxic waste.
In familial cases involving SOD1 mutations, the problem is even more direct. The SOD1 protein’s entire purpose is to disarm one of the most common free radicals produced by mitochondria. When that protein is defective, the cell’s first line of antioxidant defense is compromised from the start, shifting the internal environment toward chronic oxidative damage.
Environmental Risk Factors
For the vast majority of people with ALS who have no family history, the question of what triggered the disease is harder to answer. No single environmental exposure has been proven to cause ALS on its own, but several have shown consistent statistical associations.
Pesticide exposure stands out as one of the strongest. A case-control study measuring pollutants directly in patients’ blood found that people with ALS were roughly five times more likely to report cumulative pesticide exposure than people without the disease. Specific chemicals showed elevated risk: cis-chlordane, a now-banned insecticide, was associated with nearly five times the odds of ALS. Certain industrial compounds, including specific PCBs and brominated flame retardants, were linked to two to three times the typical risk.
Lead exposure has also been repeatedly flagged. In one study, a doubling of blood lead levels was associated with 2.6 times the odds of ALS. Elevated copper levels showed a similar pattern, with a doubling linked to 3.4 times the odds. Interestingly, higher levels of selenium and zinc appeared protective, each associated with roughly halved odds of the disease.
The Military Connection
Military veterans have long been recognized as having elevated ALS risk, and the U.S. Department of Veterans Affairs treats ALS as a service-connected disease. The reasons likely involve overlapping exposures that concentrate in military environments. Veterans are more likely to encounter lead, pesticides, chemical agents, and traumatic head injuries. Exposure to Agent Orange, the herbicide used during the Vietnam War, was associated with 2.23 times the odds of ALS in a large Korean study of military personnel exposed to herbicides. Head injuries sustained during service were linked to roughly double the risk, particularly injuries that occurred within 15 years of diagnosis. Even nerve-agent pretreatment pills given to soldiers during the Gulf War were associated with 2.7 times the odds.
Cyanobacteria and BMAA
One of the more intriguing environmental hypotheses involves BMAA, a toxin produced by blue-green algae (cyanobacteria) found in lakes, oceans, and soil worldwide. The hypothesis originated from observations of unusually high ALS rates among the Chamorro people of Guam, who were exposed to BMAA through their traditional diet. Primate studies have shown that long-term exposure to low concentrations of BMAA produces brain damage resembling human ALS, even at doses far below what would cause the acute nerve-overstimulation damage the toxin is known for at high doses. This suggests BMAA may slowly alter cell function over decades rather than killing neurons outright. The hypothesis remains active but unproven in humans, and the exact mechanism is still being worked out.
Who Gets ALS
ALS affects 2 to 3 people per 100,000 each year in Europe, with similar rates in other populations of European descent. Symptoms typically first appear around age 65, though the disease can strike much younger. The male predominance is strongest in younger age groups, with men under roughly 50 developing ALS at nearly four times the rate of women. By older ages, the gap narrows to about 1.2 to 1.4 men for every woman. This pattern has led researchers to speculate that estrogen may offer some protection, since the sex difference shrinks after the age when most women have gone through menopause.
The disease remains relatively rare, with a prevalence of 7 to 9 per 100,000 people at any given time. But projections suggest these numbers will rise as populations age, since age is the single strongest demographic risk factor. The combination of advancing age, accumulated environmental exposures, and whatever genetic susceptibility a person carries appears to determine who ultimately crosses the threshold into disease.