"All Causes Cancer": A Guide to Understanding and Assessing Risks

When it comes to talking about cancer, ignorance quickly takes over. It's easy enough to fall into alarm and misinterpret messages. Sometimes the fault is not sensationalism, but the lack of transparency in the information.

Talking about cancer is always difficult. There is no such disease called cancer, but a complex set of pathologies produced by the lack of control of the tissues. This messes everything up a lot, especially when we consider the thousands of factors that cause it. But how important are these factors? Are they all worth the same? How do they affect us? Today we are going to face these questions.

The disease called "cancer" does not exist

Although we need the colloquial license to call all pathologies associated with tumors cancer, in reality cancer is not a disease, but thousands of them. Each cancer differs from the others in many ways. The nature of the mutations, the molecular manifestation in the cells, their behavior or the ability to resist treatments are just some of the questions that determine whether it is a cancer or another.

But let's get back to what interests us. Many of these cancers are determined by another issue: the factors that promote them. Unfortunately, living is the biggest cancer-causing factor. We do not say it in a joking tone. Promoter factors can be internal, associated with metabolism and our genetic material.

On the other hand, external factors often affect these internal factors indirectly, causing long-term cancer. Other times a cancer does not develop a lethal disease, but remains in mere benign tumors or remits spontaneously in almost miraculous cases. In short, these brushstrokes are only intended to introduce a topic that, because it is monstrously large, is very difficult to tackle.

What is the risk of cancer?

Now let's get to the point first. What do we call cancer risk? The first and most important thing, and that we must be very clear: risk is equal to possibility, but it is never the same as guarantee. The clearest example is that we are all at risk of a car accident if we drive, if we ride a bike, if we go outside, if we ride a bus ...

It is becoming more and more common for the media and popularizers to use relative risk to warn us of the dangers of unhealthy practices, such as the consumption of red and processed meat. #Thread pic.twitter.com/TabXTvqpgo

- Guillermo Peris (@waltzing_piglet) June 27, 2019

Estimates of the risk of cancer and other diseases are determined by studying large groups of people. This is done by measuring the probability that a group will get cancer in a certain period of time. The risk can be absolute or relative. Absolute refers to the actual numerical probability or probability of developing cancer during a specific period.

Sometimes we talk about absolute and relative probabilities without distinction, and they do not mean the same, nor is any of them a guarantee of having the disease

The relative consists of a comparison or proportion between two different factors. It is important to understand that the relative only shows the relationship between these factors. It is not the same as a person having a 50% chance of suffering from a type of cancer that a factor increases the chances of having it by 90%. In the latter case, the relative probability (90%) would be over the absolute probability.

Let's imagine that the absolute probability, this time, is 2%. 90% of the relative probability over this 2% means that it is actually only 1.8% of the absolute value. This is important because we sometimes talk about these types of probabilities without distinction, and they do not mean the same thing. Also, remember that probability is not equal to certainty, and that probabilities are not additive, normally. This means that a probability of 50% is not added to another of 2%, etc. They are probabilities independent of each other, so they cannot be associated except in rare exceptions.

What are the factors that can cause cancer?

The more we know, the clearer we know that life habits are determining factors in the possibility of suffering from cancer. That is, a certain lifestyle increases its probability (again, it does not guarantee it), and others decrease it. Thus, we know for sure that smoking increases the chances of developing lung, mouth, esophageal cancer ... and that excessive alcohol consumption also increases the probability of liver, stomach, colon cancer ...

All of these factors are called external factors. Within these are practically 100% of those that the media echo, as they directly affect our habits. These factors can cause a mutation in the cells or produce a metabolic problem that ends up increasing the chances of cancer.

For one of these factors to be able to cause cancer, it must produce between four and six mutations in the cells. Some studies have expanded this range from one to ten. Normally, the rest of the factors are already considered in the probabilities of suffering a cancer caused by these external factors: if the substance penetrates, if it acts on the DNA, if it persists, if it is capable of causing the mutation, if the DNA is not capable of solving it ... But this is not so simple.

At other times we rely on clues and hypotheses. Sometimes only in inquiries and blind shots. In these cases tests are done with animal models and clinical studies trying to relate one of these external factors (substances, food, habits) with a probability. In many of these cases the statistics are related, but not decisive. You can't always find a direct relationship, also known as a causal one (cause), but we have to settle for a correlation: there is some kind of relationship between cancer and factor, but we don't know how it works.

This can be good or bad. Good because it allows us to be aware of the possible dangers of this factor. Bad because it could raise the alarm about a substance because of a relationship that is purely casual. In fact, this happens very often. Also, to make things more complicated, then there is that aphorism that says "the poison is in the dose". And is that a substance, per seIt is not always related to the probability of suffering from cancer, but depends on the quantity and the way of administering said substance.

How to evaluate a suspected carcinogenic substance

If we go to the supermarket we will face a lot of supposed external factors capable of raising our chances of suffering from cancer. How do we assess the hazard correctly? Everything we have discussed so far seems very complicated: probabilities, external factors, causal relationships ... to simplify the impossible, there are numerous scientific committees and several entities dedicated body and soul to summarize this information.

And summarizing is the right word because to do so they need years (more than a decade normally) of experiments and reviews of the scientific literature to finally obtain a verdict on the substance being evaluated. Among these entities, the International Cancer Research Center, or IARC, stands out.

This intergovernmental body is part of the United Nations World Health Organization. He is the maximum exponent in this matter and has a list that everyone refers to and is taken as a touchstone to assess the safety of substances in terms of cancer. This list is divided into groups, from 1 to 4, and subdividing group two into A and B.

In the face of suspicion or social alarm, or in the face of new scientific evidence, a committee of experts meets to indicate the need to evaluate a specific substance. Working groups are then set up to review all existing studies on a topic and analyze its validity and implications. This, for example, is what happened recently with red meat. After reaching a consensus, the IARC committee labels the substance in one of these groups that we discussed.

How do IARC groups work?

Group 1 labels a substance as carcinogenic, that is, we know for sure that exposing ourselves has certain absolute chances of causing cancer. Let's go back to the beginning of the article and remember that absolute probability speaks of possibilities, not guarantee. Thus, alcohol and tobacco are in group 1, along with benzene, asbestos, formaldehyde or sunlight.

And they are in this group because we know for sure that exposing ourselves to them has a risk of cancer. It should also be borne in mind that tobacco and asbestos, for example, do not have the same absolute probability of causing cancer, not to mention amounts. Although both are in group 1, it is much more dangerous to smoke, in total numbers, than to be exposed to asbestos.

Group 2 is divided into 2A and 2B. 2A is the group of "probably" carcinogens, and includes those substances for which there is limited evidence for humans (in other words, there is suspicion but there is no evidence) but there is confirmed evidence in experimental animals . Group 2B also brings together the suspected substances but, unlike 2A, there is not even evidence in experimental animals.

That said, it must be remembered that an animal model is not always extrapolated to human beings. In other words, we cannot say that because there are animal tests, a substance is going to be carcinogenic in humans (and vice versa). Group 3 includes substances whose evidence indicates that it is not possible to classify them as a carcinogen, for the moment, and group 4 those for which they exist that demonstrate that there is no association with cancer.

So what is in group 2, is it carcinogenic?

No. The only substances that we can say are carcinogenic are those from group 1. All those from group 2 can be said to be safe, but they should be taken in moderation, especially those from group 2A, and they are not dangerous. This, however, has been misinterpreted over and over again in the popular acquis.

Group 2 substances "are not carcinogenic", there is some suspicion, but no evidence that they pose a problem, so it is advisable to be exposed to them in moderation

Group two only speaks of suspicions, more or less unfounded, but never of clear evidence, which only corresponds to group 1.This classification comes from the hand of the precautionary principle, which taken out of context gives many misunderstandings. If we take into account that group 2 only talks about suspicions of the possibility that, in some cases, these substances may be related to cancer ... we will understand that "to say that these substances are carcinogenic" is irresponsible.

The problem with this classification, which we have already said is the most important, is evident: it is neither clear nor specific. Put coffee, dry cleaning and diesel fuel, or meat in the same bag with hairdressing or refined oil. However, he never talks about absolute odds, despite the alarm it raises.

Despite WHO's efforts to clarify substance reports, the media and population constantly misunderstand the IARC classification. If we want to determine what risk a substance has of being carcinogenic, we cannot limit ourselves only to these lists, but we will have to look for concrete information on the statistics and absolute probabilities (something that is not always easy).

But if we only rely on the IARC recommendations, what we should do is read the organism's reports, simple sheets where everything related to the evaluation is explained. As a general note, it is good to remember that it is best to avoid, as much as possible, what is contained in group 1 and to expose ourselves in moderation to group 2; without obsessing and understanding that the subject is much more complex than a list will ever be able to reflect.

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