This ubiquitous element almost turns all of humanity into fools

Produced by: Science Popularization China

Author: Zhang Hao

Producer: China Science Expo

In March this year, a study published in the authoritative journal Proceedings of the National Academy of Sciences pointed out that the widespread use of leaded gasoline had a negative impact on the intelligence level of more than half of the population in the United States, and the average IQ of each person dropped by 2.6 points.

Today, let's talk about the story of "lead". Before I start the main text, I want to repeat something that everyone is familiar with, that is, the pencils we use, although called pencils, have nothing to do with lead. The composition of the pencil core is graphite, which is a form of carbon. So even if you like to "bite the pen tip" when you are a child, don't worry.

Back to the topic. Coincidentally, in a 2020 study published in the Journal of the American Medical Association, scientists scanned the brain structures of more than 1,000 45-year-old adults born in the same town in New Zealand, who grew up during the peak period of leaded gasoline use. The study said that for every 5 micrograms/dL of lead carried in the subjects' bodies at age 11, their cerebral cortical surface area was 1 square centimeter smaller and their hippocampal volume was 0.1 cubic centimeters smaller than the control group at age 45. Their IQ was 2 points lower on average, and some people also showed cognitive impairment.

Among them, participants with the highest lead exposure also showed damage to the integrity of white matter in their brains. Since childhood is a critical period for brain development, lead exposure during this period can have long-term effects on the brain and may even lead to degenerative diseases in old age.

In our daily lives, there are many things that we take for granted but actually cause serious damage to the human brain. For example, lead pipes were once popular, and the leaded car exhaust mentioned above. Although these lead pipes and leaded exhaust are slowly fading out of human life, the harm they cause is like a shadow, lingering around many victims for a long time.

Leaded gasoline has been popular for nearly 100 years, which is certainly due to compromise and helplessness, but the capital forces and regulatory failures behind it are also to blame. Why is lead added to gasoline? What symptoms will people suffer from long-term inhalation of leaded exhaust? Can humans completely eliminate lead pollution in the future and even exclude lead from commonly used materials?

Lead crystals and processed lead blocks (Image source: Wikipedia)

How did leaded gasoline additives come onto the stage of human history?

In the process of continuously improving internal combustion engines, humans gradually discovered the so-called "detonation" phenomenon. In the early days of internal combustion engines, the compression ratio was low and the temperature in the cylinder was not high. As long as the spark plug did not ignite, the oil-gas mixture in the cylinder would not burn. When we pursue higher performance internal combustion engines, we inevitably have to increase the compression ratio and increase the cylinder operating temperature. At this time, part of the oil-gas mixture may spontaneously ignite before the spark plug ignites, which is gasoline detonation.

This is like when everyone is working together to race a dragon boat, but someone doesn't listen to the call and rows on his own, which will inevitably affect the entire team. Detonation is definitely not a good phenomenon for an internal combustion engine. It not only reduces the fuel efficiency of the internal combustion engine, but may also make noise and even damage the cylinder. To solve the detonation problem, of course, we can start from the structure of the internal combustion engine and the gasoline itself, but the development of the internal combustion engine structure had reached a bottleneck at the time, and increasing the compression ratio was an inevitable way in the development of the internal combustion engine. Therefore, engineers could only think of ways to solve the problem with gasoline.

Midgley (left), the inventor of leaded gasoline additives, and his product packaging (right)

Image source: Wikipedia

Soon, Thomas Midgley, who worked for General Motors in the United States, discovered in 1921 that a compound called tetraethyl lead could effectively suppress gasoline knock. The synthesis process of this substance is simple, the cost is quite low, and only a small amount of addition is needed to effectively improve the anti-knock performance of gasoline. Unfortunately, lead and some lead compounds were already well-known poisons at the time, and the introduction of leaded gasoline additives to the market was likely to cause public concern.

However, General Motors showed great interest in this technology. In order to successfully launch the product, they deliberately concealed the fact that gasoline contained lead when naming the product, calling it "ethyl gasoline additive". Since the construction of the production line, many workers have become ill or even died due to lead poisoning. However, in the face of the huge economic benefits of anti-knock gasoline additives, General Motors still ignored various safety hazards and promoted tetraethyl lead to become the most widely used gasoline additive in the next 100 years.

Leaded exhaust

The hazards of leaded gasoline additives soon began to emerge. In the production process, workers suffered from acute poisoning and chronic occupational poisoning frequently. In the consumption process, a large amount of lead waste entered the atmosphere with the emission of automobile exhaust and accumulated in areas along the road. Developed countries entered the automobile society earlier, and the number of motor vehicles was far ahead of developing countries decades ago, so they became victims of lead exhaust pollution earlier.

Image source: veer gallery

Among the acute poisoning incidents caused by tetraethyl lead gasoline additives, the most famous and dramatic one is the trial by its inventor Midgley. On October 30, 1923, in order to dispel the public's doubts about the safety of this substance, Midgley held a press conference and took out a small bottle of tetraethyl lead additive on the spot, smeared it on his hands, and sniffed the bottle for 60 seconds. Who knew that this press conference turned out to be a "large car accident scene". A few days later, not only was a tetraethyl lead plant under General Motors shut down by the New Jersey government, but Midgley himself also showed severe symptoms of lead poisoning. After he was sent to the hospital, he took a full year of recuperation before he basically recovered.

In 1970, a serious case of chronic lead poisoning also occurred in Yanagimachi, Tokyo. Yanagimachi is a neighborhood in Shinjuku, Tokyo. At the time, it was the intersection of several main roads, and it was located at the bottom of a valley, so air easily accumulated. Car exhaust encountered the "valley bottom" terrain, making the carbon monoxide content in the air in the area among the highest in Tokyo all year round. This led to a high incidence of respiratory diseases among local residents, and there was even a saying of "Yanagimachi asthma".

Car exhaust in winter (Photo source: veer photo gallery)

In order to thoroughly investigate the health status of local residents, medical institutions conducted detailed physical examinations on local residents. The results showed that the blood lead content of the residents tested here reached an average of 47.7 mg/dL, with a maximum of 138 mg/dL, which is basically more than 7 times that of other parts of the country. At that time, the standard for occupational lead injury stipulated by the Ministry of Health, Labor and Welfare of Japan was 60 mg/dL, and many residents of Yanagimachi were already in a state of lead poisoning.

However, the impact of chronic lead poisoning on the nervous system is difficult to describe with specific numbers, after all, this may be a slow process that lasts for decades. Although the above research data has a series of details that are worth pondering, it can generally reflect the long-term and far-reaching toxicity of lead poisoning to the human nervous system. So, what exactly does the element lead do to the human body, why does it affect human health, and what are the symptoms and treatments of lead poisoning?

What kind of element is lead?

Lead is a widely existing metal element in nature. Single lead has the characteristics of high density, easy melting and processing, and is widely used in industry and daily life. The density of lead is about 1.4 times that of iron, so single lead is often used to make various forms of counterweights. Various lead compounds have become an indispensable raw material for mankind since the Industrial Revolution. In addition to gasoline additives, the major industrial uses of lead include lead-acid batteries, tin-lead solder, lead paint, lead alloy jewelry, etc.

Lead bricks used as radiation shielding materials (left) and small lead-acid batteries (right)

Image source: Wikipedia

As a toxic element, lead poses no risk of poisoning in its elemental form as long as it does not enter the body. It is very safe to touch lead in bulk form, but it is very dangerous to ingest it. Elemental lead will dissolve into lead ions under the action of stomach acid. Once these ions enter the blood, they will quickly flow throughout the body and cause damage to multiple organs.

In 2006, a 4-year-old child died in the United States after accidentally ingesting pure lead jewelry. A 4-year-old boy with mild intellectual disability went to the hospital due to sudden vomiting. The doctor did not initially suspect that the boy had accidentally ingested poison, and only prescribed antiemetics before letting the parents take the boy home. Unexpectedly, two days later, the boy's condition suddenly worsened, with severe abdominal pain and weakness in his limbs. After being sent to the hospital, the boy developed a series of neurological symptoms such as excitement, difficulty breathing, and brain edema. X-ray examinations showed that there was a foreign body in the boy's stomach.

Four days after being admitted to the hospital, the lead concentration in the boy's blood reached 180 mg/dL, and the doctor suspected the possibility of acute lead poisoning. But it was too late. The boy showed symptoms of brain death that night and died five days after being admitted to the hospital. The postmortem autopsy results showed that the foreign body in the boy's stomach was a heart-shaped decoration on his boots, which contained 99.1% lead.

After this tragedy, many countries conducted multiple inspections on cheap metal jewelry on the market, and the conclusion showed that lead and lead alloys are still used by some manufacturers to make low-cost jewelry because of their low price and solid texture. These jewelry will not cause any health effects during daily wear, but if they are accidentally ingested by children, there is a great risk of poisoning.

Examples of lead poisoning: (left) Lead plumbing system, (right) Scavengers are susceptible to poisoning after eating prey shot with lead bullets

Image source: Wikipedia

Lead compounds are more likely to cause environmental pollution

After talking about the single element of lead, let's take a look at the health risks of various lead compounds (including lead ions dissolved in lead water pipes) to the human body. Although various inorganic lead compounds can exist stably outside the body, once they enter the body through the respiratory tract and digestive tract, they will gradually deposit and slowly decompose. The decomposed lead ions are most abundant in the blood, soft tissues and bone tissues. Among the various organs, the kidneys and liver are involved in the metabolism of lead, so the lead content is higher, while the lead content in the brain is relatively much lower.

Gaseous organic lead compounds released from automobile exhaust enter the human body mainly through the respiratory tract. Due to their fat solubility, the skin is also a possible route for them to enter the human body. Although lead will accumulate after entering the human body, it can also be excreted to a certain extent. Feces, urine and even breathing are all ways for lead to be excreted from the body. Unlike inorganic lead compounds, the lead concentration in the brain will increase significantly after inhaling organic lead compounds, which is one of the important reasons why lead-containing exhaust can easily affect the nervous system.

The impact of lead on the body: (left) People with a history of lead exposure in childhood have a significantly smaller prefrontal lobe in adulthood (red part), (middle) special epiphyseal lines after lead poisoning, (right) basophilic stippled red blood cells after lead poisoning

Image source: Wikipedia

The main reason why lead causes abnormalities after entering the human body is that it can interfere with the synthesis of hemoglobin. When the concentration of lead in the blood reaches 40 micrograms per deciliter or more, the biochemical process related to hematopoiesis will begin to be disturbed, but at this time the abnormality in the body is not enough to develop into obvious symptoms that can be detected in the short term. When the blood lead concentration reaches 60 micrograms per deciliter or more, the indicators of multiple organs such as the liver and kidneys in the body may be abnormal. When the blood lead concentration further rises to 80 micrograms per deciliter, systemic symptoms begin to appear, the primary manifestations of which are anemia and a unique grayish-white face, and the nervous system will also show various manifestations such as headaches, sensitivity, and mania.

When the blood lead level in the body rises to more than 150 micrograms per deciliter, the human body is already in a state equivalent to acute lead poisoning, and major body systems, especially the nervous system, will show serious abnormalities. The normal function of the human brain is difficult to maintain, and the motor and sensory nerves are abnormal. Under the effect of nerve paralysis, patients often show unique symptoms of drooping hands.

Beware of child killers (Photo source: Changsha Evening News)

In recent years, studies have repeatedly found that long-term exposure to an environment with excessive lead content, even if the blood lead concentration does not reach the traditional disease-causing risk value, damage to the human body, especially damage to the nervous system, has already occurred. Therefore, this study does not propose a new conclusion, but only adds some new evidence to the long-standing view. Unlike other metal elements such as copper and iron, lead has no known effect in the human body and is a purely toxic and harmful substance. However, due to the extremely wide distribution of lead in nature, humans will inevitably ingest lead through various channels, mainly through diet.

The daily lead intake of Chinese residents varies from region to region. Taking Hangzhou residents as an example, a 2014 survey showed that the lead intake of Hangzhou residents was about 80 micrograms per day. This figure is quite far from the tolerable intake of 200 micrograms per day, but it is still significantly higher than neighboring countries such as Japan (about 20 micrograms) that have implemented comprehensive control of lead pollution earlier.

With the introduction of more stringent environmental protection measures, it is believed that the lead intake of Chinese residents will gradually catch up with the world's advanced level. Therefore, it is more important to take more comprehensive "lead ban" measures in various industrial sectors.

The presence of lead determines whether it is harmful to the environment

In fact, what determines the toxicity of lead to the human body and the environment is the presence and distribution of lead, which directly determines whether lead-containing materials can be widely promoted and applied. For example, although nearly 90% of industrial lead is used in lead-acid batteries, the possibility of lead compounds being released to the outside world and polluting the environment is very low, so lead-acid batteries have always been the first choice for manufacturing car batteries and UPS backup power supplies. However, tetraethyl lead, an additive for anti-knock gasoline, and tin-lead solder used in the electronics industry pose great environmental risks.

Although tin-lead solder has excellent performance and low cost, the lead alloy on the waste circuit board cannot be recycled due to the many solder joints on the circuit board. The huge consumption and waste of electronic products make it impossible to centrally treat them harmlessly. Ultimately, the circuit boards piled up in the wild garbage disposal site may release lead into the soil and groundwater continuously under natural conditions. The lead will eventually be enriched by plants and aquatic animals and then enter the human body through the food chain.

Waste circuit boards (Photo source: veer gallery)

Considering the potential environmental hazards of improper use of lead, even though tetraethyl lead and tin-lead solder have incomparable performance advantages, humans must say goodbye to them. At present, only a few countries and regions in the world still sell leaded gasoline, and after nearly 30 years of efforts, the electronics industry has basically eliminated the use of tin-lead solder in the field of civilian consumer electronics. These measures have effectively curbed the rising trend of lead content in the environment and reduced the amount of lead that humans ingest through breathing and eating.

Looking at the history of the earth's evolution, we can see that every element is a treasure given to mankind by nature, and they always create opportunities for human development at some point. Although some forms of lead and lead compounds have certain human toxicity, they are still irreplaceable key materials in many occasions. Moreover, as long as they are used in a scientific way, these lead-containing materials will not cause harm to the human body and the environment. In the future, humans still need to more strictly control the entry of dispersed forms of lead into the environment, and at the same time, they need to more reasonably develop and utilize lead so that these resources can truly benefit mankind.

References:

Half of US population exposed to adverse lead levels in early childhood

https://www.pnas.org/doi/full/10.1073/pnas.2118631119

One of the worst public health dangers of the past century has finally been eradicated

https://www.vox.com/future-perfect/22650920/leaded-gasoline-eradicated-public-health

Lead exposure in the last century shrank IQ scores of half of Americans, study finds

https://www.sciencedaily.com/releases/2022/03/220307162011.htm