For more than two decades in the back pages of the New Scientist, readers have posed questions on topics that have confounded them and each week the magazine has published the best answers supplied by its readers. Here we publish some of those answers to the mysteries of the human body.

Why don’t we sneeze in our sleep?

A sneeze is a reflex reaction, an involuntary response to an internal or external stimulus. Reflex reactions shut down during sleep because of rapid eye movement (REM) atonia, a physiological state during that movement messages can’t get from the brain to the muscles, causing partial paralysis. This prevents sleepers from acting out their dreams to the detriment of themselves or their bed partners. A consequence is that reflex signals are not acted upon, so there’s no sneezing during sleep. If a sneeze is absolutely necessary, REM atonia ceases and the person wakes up before sneezing. There have been cases of REM sleep behaviour disorder in which people acted out their dreams and ended up in court after injuring or killing their partner. Presumably such people can sneeze in their sleep.

David Muir, science department, Portobello high school, Edinburgh

Why do loud noises seem to hurt? They don’t seem to cause physical pain but do induce mental anguish. I heard a truck taking on board a bin of empty bottles and the noise was excruciating. I had to get out of earshot.

Two factors contribute to the unpleasantness of the noise from bottles going into a truck: the volume and the discordant frequencies. The human ear detects sound using tiny hair cells in the cochlea. A soundwave causes mechanical deformation of these cells, sending an electrical signal to the brain via the auditory nerve. Different groups of hair cells react to different frequencies, with hairs detecting low notes at one end of the cochlear spiral and high notes at the other. High volumes can damage the delicate hearing apparatus from chemical exhaustion of the cell-signalling process and mechanical damage.

We are adapted to find damaging stimuli unpleasant and painful so that we avoid them. Humans also find notes that are too close together very unpleasant – think of the discord between two adjacent notes on a piano compared with reasonably pleasant sounds obtained by playing notes spaced apart. This is probably because the closeness of the frequencies means there is an overlap in the groups of hairs activated by each note, overstimulating those that can sense both. Glass colliding as it is dumped into the truck creates many discords at once. For high notes like the crash of breaking glass the problem is worse, because as the notes get higher the difference between adjacent notes gets smaller and more hairs will be stimulated by a given note. We are therefore more likely to experience discordance at higher frequencies.

Miriam Ashwell, London

Alcohol can protect you against microbes, one study has found. Photograph: Alamy

If you have a few drinks, does the alcohol in your bloodstream act as medication against any microbes in the body?

There can be a protective effect, but only if the alcohol concentration is high enough. A study in the journal Epidemiology is one of the few to confirm the benefits of drinking alcohol with a meal. During an oyster-borne outbreak of hepatitis A, those drinking alcohol of 10% strength or higher experienced a protective effect, although this would have occurred in the stomach, not the bloodstream.

I would argue that for the sake of your health, total daily consumption should be moderate, the equivalent of two glasses of table wine.

Lewis Perdue, Sonoma, California

My thoughts feel as though they are in my head. Is there a physical basis for this or is it just that I know that’s where my brain is?

Your head is the focus for receiving information from your environment, mainly through sight and sound. Thought is secondary to sensory perception, but both have to combine quickly when you are threatened, and evolution has sited the organs for sensing information and processing it close together to ensure speedy response to dangers such as sabre-toothed tigers and muggers.

Researchers at the Karolinska Institute in Stockholm, Sweden, investigated out-of-body illusions using a brain scanner. They found that the brain’s posterior cingulate cortex combines the feelings of where the self is located with that of body ownership. This is why you feel as if your thoughts are inside your head. Sensory deprivation can cause such feelings to be lost. When people lose sensory awareness while they are still conscious they become disoriented and may experience out-of-body sensations, feeling as though their thoughts are no longer inside their head. This can occur during certain types of torture or can be self-induced through meditation or drug use.

David Muir, science department, Portobello high school, Edinburgh

We hear a lot about bugs, flies and fungi gaining resistance to pesticides and other chemical treatments, making such solutions futile. If this is the case, shouldn’t humans gain resistance to threats such as harmful chemicals and bacteria?

Humans accumulate genetic changes all the time, as has become obvious through whole genome sequencing studies. These can range from as little as a single base substitution – a change to one “letter” of the 3bn pairs in your genome – to alterations to whole chunks of DNA. But because the human genome is so big and complex, many of these changes have no discernible biological consequences.

Known adaptations to threats include certain populations that have retained the genes for blood diseases such as thalassaemia and ovalocytosis, because these confer some resistance to malaria, a more serious threat. However, if an advantageous mutation occurs, it can take a long time to spread through the population, because humans live for many years and produce relatively few offspring. This generation time has a key role in determining the rate at which a genetic change gets established in a population, but genome size is also crucial.

As organisms become less complex down the evolutionary tree, their genomes become smaller. Our genetic material encodes approximately 30,000 genes known so far. Insects, such as the mosquito that transmits malaria or yellow fever, have just over 12,000 genes. Parasites, such as malaria, are unicellular organisms that have around 5,300 genes. Bacteria are even smaller.

The relevance of this is that the smaller and simpler the genome, the more likely it is that changes will have biological consequences. This means that random changes can be more significant and make a survival difference, but for the change to have an impact in the population, it has to be propagated through subsequent generations.

The propagation of genetic changes depends very much on the lifespan of the individuals and on how many offspring they can transmit this change to. Humans commonly have no more than a few offspring in their lifetime, whereas mosquitos can lay hundreds of eggs. Unicellular parasites, such as malaria, have a life cycle of 48 hours in the blood. During this time they can proliferate exponentially, so any genetic change that gives a survival advantage spreads very quickly.

This is why parasites such as malaria are notorious for developing resistance to drugs and why mosquitos become less sensitive to insecticides, but we humans are slow to adapt to our own biological threats.

Alena Pance, the Wellcome Trust Sanger Institute, Cambridgeshire

The humble hamburger: so much more appealing (to most of us) than lettuce. Photograph: Rex

Do humans have an innate desire to eat meat or is it cultural? If our culture had all references to eating meat removed, would people still desire it?

It is less an innate desire for meat in particular than for concentrated nutrition such as fats and proteins. Even in non-meat foods, we favour concentrated items such as nuts, fruits, grains and tubers. Think of how appetising chocolate or halva are compared with lettuce or grass.

Herbivores generally must eat large quantities of low-grade plant foods, discarding most of the fibre and excess materials they contain. They also discard the toxins; plants are generally full of harmful chemicals that herbivores must tolerate, discard or destroy, while concentrating the vitamins, proteins, fats and digestible carbohydrates.

The resultant purified herbivore flesh suits carnivores that would die if they tried eating too much of the wrong plant matter; the list of wholesome plant feed that could kill your dog or cat is shocking. Many herbivores are partial to a bit of meat if they can get it and omnivores generally will work harder for animal food than plant food.

So it is with us; most of us relish and thrive on some of the concentrated fat, protein, vitamins and essential fatty acids in meat.

Jon Richfield, Somerset West, South Africa

Why is the scrotum so wrinkly?

The scrotum plays a valuable role in thermoregulation of the testicles, as sperm production is reduced at core body temperatures and above. Those of us with this piece of anatomy will have noticed that on a very cold day, or when emerging from a cold shower, the scrotum (and the rest) will be much smaller. The converse happens on a warm day or on emerging from a hot bath. The scrotum is quite smooth when relaxed and only wrinkles as it is pulled tight to the body.

This is because muscles within the scrotal wall contract to bring the testicles nearer the abdomen in cold conditions and relax to keep them cool when it is hot. There is also a complex system of vasculature in the spermatic cord (the pampiniform plexus), where warm blood leaving the abdomen is cooled by a heat exchanger system before entering the testicles.

When farmers first started to rear merino sheep in Australia, they found that the rams were sterile until sheared, so their testicles are shorn on a regular basis today. This breed of sheep originated in southern Portugal, where summer temperatures are not as high. Consequently, the rams have unsuitably woolly scrotums for southern Australian conditions.

Bob Butler, retired veterinarian, Llangoed, Anglesey

While on the scales this morning I wondered, would passing gas affect the weight of the human body at sea level and, if so, in which direction?

Flatulence is mostly methane, hence the schoolboy trick of putting a match to it. Methane (molecular weight 16) is lighter than air, which is approximately 80% nitrogen (molecular weight 28) and 20% oxygen (molecular weight 32). So you will gain weight on the scales when you pass wind. The odoriferous components are all denser (the lightest would be hydrogen sulphide, molecular weight 34), but are not likely to be present in sufficient quantity to make much difference.

Burping after a fizzy drink will have the opposite effect, because you are expelling carbon dioxide (molecular weight 44), which is denser than air. So your weight on the scales will decrease.

Guy Cox, school of medical sciences, University of Sydney, New South Wales, Australia

Sickle-cell anaemia is an oft-cited example of a human genetic change that confers a benefit as well as a disadvantage. Photograph: Getty Images

The only genetic changes in humans we ever hear about are those producing diseases such as cystic fibrosis. Has anyone identified any genetic changes within recent generations that make individuals possessing them “more fit” to thrive in today’s environment? Would we even know these changes if we saw them? And would we consider them normal for healthy humans?

The example that is most often cited of a positive genetic change within a recent generation is that of sickle-cell anaemia, which appears to give an advantage to people who have it in places where malaria is endemic. While we wouldn’t necessarily consider having sickle-cell anaemia as a healthy trait, carrying a copy of the “faulty” gene confers a level of resistance to malaria and is therefore selected for in populations exposed to malaria parasites. This is generally taught in most high-school biology courses as an example of human natural selection, as well as the idea that a trait is neither positive nor negative intrinsically: rather, its value is dependent on the environment.

However, another example of a positive trait that is necessary for survival in today’s environment is our adjustment to being able to cope with the high level of sodium in our diet. In populations that have a low daily salt intake, such as some indigenous Amazonians, people can retain salt effectively; they lose less sodium in their sweat and urine compared with average citizens of the US. If they are subsequently exposed to a high-sodium diet, however, this ability to retain salt works against them and many may suffer from hypertensive disorders and die at a very young age from heart disease.

Mark Bilger, Livonia, Michigan, US

Although it seems to be frowned on in polite company, is it in fact necessary to pick your nose to keep it clear and functional? Nobody likes their nose to be dirty or uncomfortable, so why is nose-picking such a taboo area in many cultures?

I don’t imagine it is necessary to keep nasal passages clear at all times. The annoyance experienced by people with a partially blocked passage suggests a deeper story. Somewhere in our past, there may have been nasal parasites (adult, larvae or eggs) we would have been smart to eliminate before they travelled further into the sinuses. A clean nose was a safe one.

There’s a stronger case for the taboo of nose-picking. It fits with a more general taboo against bringing hidden bodily tissue or products into sight. We swish saliva around our mouths all the time, yet react with disgust when it emerges as spit or drool. And it’s the same for earwax or dandruff.

“Wrong things in the wrong place” can cause contagion and they are full of microbes, even if not necessarily pathogenic ones. You don’t have to be in danger of contracting Ebola to be wisely wary of bodily fluids and nasal contents.

Louise Fabiani, no address supplied

Extracted from How Long Is Now, published by John Murray (£7.99) on 20 October. Click here to order a copy for £6.55