January 28, 2008

Lead Exposure Accelerates Mental Decline

New research suggests that some of what we now consider to be normal aging in the brain might in fact be the long-term effects of exposure to environmental pollutants, such as lead, many years ago.

"We're trying to offer a caution that a portion of what has been called normal aging might in fact be due to ubiquitous environmental exposures like lead," Dr. Brian Schwartz of John Hopkins University told Associated Press.

"The fact that it's happening with lead is the first proof of principle that it's possible," said Dr. Schwartz, a leader in the study of lead's belated effects.  

Other pollutants, like mercury and pesticides, may do the same thing, he added.  Although experts say there is insufficient research on mercury, recent studies suggest pesticide exposure raises the risk for Parkinson's disease a decade or more after the time of exposure.  

The idea that health effects of exposure to certain toxins can occur many years after the exposure is not new.  It is commonly accepted that exposure to tobacco and asbestos can lead to cancer many years, or even decades, later.  However, medical experts are now beginning to believe this might be the case with other pollutants as well.

"It's an emerging area" for research, Dr. Philip Landrigan of the Mount Sinai School of Medicine New York told Associated Press.  "It makes sense that if a substance destroys brain cells in early life, the brain may cope by drawing on its reserve capacity until it loses still more cells with aging," he said.  Only then would symptoms like forgetfulness or tremors appear.

For example, infant mice exposed to PCBs initially show only small, subtle effects.  However, by old age significant damage in areas like movement and learning is observed, said Linda Birbaum, director of experimental toxicology at the U.S. Environmental Protection Agency, in an interview with Associated Press.  

She said that studies in animals revealed clear evidence that exposure to harmful substances while in the womb can damage health later on in life.  And rodents exposed to PCBs or other dioxins before birth are at greater risk of developing cancer once they grow up.  

But studying these types of delayed health effects is more difficult in humans because they must take place over a long period of time.  However, research with lead is easier since scientists can quickly measure accumulated lead in the shinbone and determine how much lead a person has been exposed to in the past.   

Whereas the shinbone measurement provides an indication of lifetime lead exposure, blood levels of lead are an indication of recent exposure. Virtually every American will have some levels of lead in their blood, albeit at much lower levels today than in the previous times, primarily due to the phasing out of lead in gasoline from 1976 to 1991.  That, along with other measures to reduce lead, has caused the average level of lead in the blood of American adults to fall 30% by 1980 and 80% by 1990.

However, recent studies suggest the long-term health effects of the high-lead era are still being felt.  In 2006, Dr. Schwartz published a study involving about 1,000 Baltimore residents aged 50-70, old enough to have been exposed to the higher levels of lead before it was removed from gasoline.  Dr. Schwartz said the participants likely received their peak lead doses in the 1960s and 1970s by inhaling air pollution from vehicle exhaust or other environmental sources.  

The scientists estimated each person's lifetime exposure by scanning their shinbones for lead.  The results showed that the higher the lifetime lead exposure, the poorer the performance across a wide variety of mental functions, such as verbal and visual memory and language ability.  

The difference in mental functioning was equivalent of aging two to six years.  "We think that's a large effect," said Dr. Schwartz.

Other studies have shown similar results.  Dr. Howard Hu of the University of Michigan led a study in 2004 of 466 men whose average age was 67.  The men took a mental-ability test two times at four-year intervals.  Researchers found those with the highest lead levels in their shinbone scan showed more decline between exams than those with lower levels of lead.  In this particular study, the effect of the lead was equivalent to about five years of aging.

While no one is claiming lead is the only source of age-related mental decline, it does appear to be one of several factors involved, said Dr. Hu.

Bradley Wise of the National Institute on Aging told AP that many causes are thought to play a role in mental decline, such as high blood pressure, diabetes, stroke, emotional stress and perhaps even education level.  These recent studies suggest that exposure to environmental pollutants, such as lead, might be added to that list.  However, they do not conclusively prove such a link.  

"I think many things impact how we age, but I think right now it's maybe premature to be giving lead a huge role in our age-related cognitive decline," said Dr. Margit Bleecker, director of the Center for Occupational and Environmental Neurology in Baltimore, in an AP article.  She called the hypothesis "a very interesting idea" deserving of more study.

Other experts were more convinced though.  "The new evidence from these studies should concern people," epidemiologist Andrew Rowland of the University of Mexico told AP.  "These two research groups are finding adverse effects on the aging brain at low levels of lead exposure."  He said more work lies ahead, but that the studies are raising important questions.

Scientists still face some basic questions about the delayed health effects of lead.  For instance, when does the lead actually harm the brain? And does a high lead level measured in a shinbone merely identify those most exposed or those who were just more harmed by the lead?  Also, does the lead in the bone continue to do harm over a lifetime, moving into the bloodstream and continuously assaulting the brain?

Just as the mechanism behind the way lead impairs the brain is still unknown, so is the question of whether or not anything can be done to help people who have already absorbed a high amount of lead during their lifetime.   A medical procedure known as chelation can remove lead from the body, but wouldn't help in the case of those with long-term exposure, according to experts.  

However, the experts do share some suggestions.  In younger people, prevention is the best strategy, according to Dr. Hu.  He urged tougher federal standards on occupational lead exposure.   

Rowland said there should be efforts to remove lead from old houses in low-income neighborhoods, since many still have lead paint.   "It's there on the walls, on the radiators, underneath the top layers of paint," he said.  "In places where the paint is crumbling, there's still exposure going on."

Another question relates to whether or not those born after the 1970s really have to worry about lead exposure at all.  

"Kids who grew up in the 21st century have a lot less to worry about," said Dr. Hu.  "But it's hard for me to be totally optimistic the current generation is completely scot-free."
In the U.S., anyone younger than 30 has seen much less exposure than older adults.  However, Dr. Hu stressed that there's still lead in the environment, and exposure remains particularly high in developing countries.  Since lead can cross the placentas, there is a chance that women who grew up in the 1970s might dose their fetuses with lead.