Dear Doctors: My understanding is that nicotine and tar are what hurt you when you smoke cigarettes. But you recently said there’s also carbon monoxide in cigarette smoke, and that it’s even more dangerous. Can you please explain? I want my husband to quit, and this may help.

Dear Reader: Carbon monoxide is a colorless and odorless poisonous gas. It is produced when anything that contains carbon is burned. It is often referred to simply as CO, which is its chemical formula. This indicates that each molecule consists of one carbon atom and one oxygen atom. (And for our fellow science nerds, the carbon and oxygen atoms of CO are connected by a triple bond.) The most common source of CO is the incomplete combustion of fuels such as gasoline, natural gas, heating oil, propane, kerosene, wood and charcoal. And, yes, the act of burning tobacco also results in the release of carbon monoxide.

What makes CO dangerous is its enormous affinity for hemoglobin. That’s the protein in red blood cells that carries oxygen from the lungs to the tissues throughout the body. A molecule of hemoglobin has four binding sites, each of which can carry a single molecule of oxygen. Due to its chemical structure, CO binds to hemoglobin 200 to 300 times more readily than oxygen. That means that when CO is present in the air someone breathes, the CO will crowd out the oxygen. This greatly diminishes the amount of oxygen the blood can carry to the cells. Even a small amount of CO can leave the cells of the body starved for oxygen.

When someone smokes a cigarette, the burning tobacco, which contains carbon, emits CO. Because the smoke is inhaled directly into the lungs, it leads to a dramatic decrease in the amount of oxygen the tissues will receive. Every time a smoker inhales, they put their body into an increasing state of oxygen debt. Among the many ill effects are breathlessness, diminished lung capacity and permanent injury to the alveoli, which are the hundreds of millions of minute air sacs in the lungs.

Exposure to CO damages the heart muscles and impairs the ability of the circulatory system to function properly. It also is linked to poor mood and cognitive impairment -- and even to sexual dysfunction. Over time, the effects of tobacco use have been shown to do harm to virtually all of the tissues and metabolic functions in the body.

It has been several decades since the link between tobacco use and disease and death became clear. Yet smoking continues to be the cause of death for 480,000 Americans each year. It causes 80% to 90% of all lung cancer deaths and 80% of deaths from chronic obstructive pulmonary disease. The habit can double your risk of stroke, and it significantly increases the risk of heart attack. It is a grim way to end this column, but this is a difficult subject. We know that quitting is difficult. But the details of this discussion show that not quitting can lead to circumstances that are even more difficult.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctors: A few weeks ago, one side of my dad’s face became paralyzed. He couldn’t close his left eye, and the left side of his mouth wouldn’t work. We thought it was a stroke, but the ER doctors said it is Bell’s palsy. They said it eventually goes away. What causes it? How long does it last?

Dear Reader: Bell’s palsy is a neurological disorder that occurs when something interrupts the signals from the facial nerve, which animates the muscles in the face. Known as cranial nerve VII, it also plays a role in taste and sensations in the ear. As happened with your father, this condition appears suddenly, can progress rapidly and usually affects just one side of the face. For that reason, it is sometimes mistaken as the effects of a stroke. It is possible, but rare, for Bell’s palsy to affect both sides of the face.

Symptoms include sudden weakness, or even paralysis, of the facial muscles. This causes the eyebrow and mouth to droop and the cheek to sag. The individual may be unable to close the eyelid on the affected side, which can cause excessive tearing. The sense of taste can be affected, and the lack of motor control of the muscles of the mouth may lead to drooling. Some people develop unusual sensations in the face, and can become sensitive to loud sounds. Episodes of Bell’s palsy can last from a few weeks to six months or more. It is possible, but not common, for the effects to become permanent.

There is no specific test for Bell’s palsy. Diagnosis begins with a physical exam and medical history. It also includes tests, including blood tests and imaging scans, to rule out other possible causes of facial paralysis. These can include physical trauma, stroke, autoimmune disease or adverse drug reactions or interactions.

Bell’s palsy is the most common cause of facial paralysis. Although the exact cause of the disorder remains unclear, there appears to be a link to the functioning of the immune system. Risk factors include high blood pressure, diabetes, pregnancy, being overweight, having a dormant viral infection, autoimmune syndromes and living with an upper-respiratory ailment. Some researchers suspect chronic inflammation, which can damage the tissues that insulate the facial nerve, may also play a role.

It has been found that starting treatment with steroids within the first three days of the onset of the symptoms of Bell’s palsy can reduce inflammation, and perhaps speed the return of nerve function. When steroids are not an option, antiviral medications may have a similar positive effect. It is equally important for treatment plans to address each person’s specific symptoms. This includes the use of lubricating eye drops or an ointment to keep the surface of the affected eye moist. An eye patch will protect the eye, particularly the cornea, from debris and possible injury. If needed, analgesic medications may be prescribed for pain. Some patients find physical therapy, facial massage or alternative therapies such as acupuncture can help manage discomfort and ease symptoms.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctors: I have been dealing with toenail fungus on just a single toe for more than a year. There is a mountain of misinformation about it on the internet, and I keep seeing ads for all sorts of drugstore products that claim they can cure it. Any suggestions for how to deal with this?

Dear Reader: You are asking about a topic that, each time we have written about it in the past, brings a flood of mail. Toenail fungus is a common -- and very stubborn -- infection. Also known as onychomycosis, is estimated to affect between 6 million to 7 million people in the United States. Although the effects are visible on the exposed part of the nail, the fungi that cause the infection are housed within the nail bed. And because our nails are so well-engineered to shield the tips of our toes from potential threats such as moisture and trauma, the fungi that take up residence beneath them are largely protected.

For those who are unfamiliar, symptoms include nails that gradually become discolored, often with a yellowish, brown or gray hue. They also become thickened and malformed, often with a brittle or crumbling texture. As the infection progresses, the nail plate can grow so thick that it becomes difficult to trim. Sometimes the infected area will also exude a musty scent. Once a fungal infection takes hold, the dark and moist environment provided by socks and shoes can help it spread to the neighboring toenails.

When it comes to treatment, the options are somewhat limited. The topical nonprescription ointments and liquids you have been seeing can make the nail look better. However, the protein that makes up the nail plate, which is known as keratin, is not porous. That means topical medications can’t reach the entire site of the infection. Our readers have reported success with home remedies, such as the use of tea tree oil and Vicks VapoRub.

A more effective approach has been an oral antifungal medication, which requires a prescription. The medication enters your blood stream and is delivered to the network of blood vessels that serve the nail bed. As with many medications, antifungals have potential side effects. These include headache and stomach upset. These meds can also affect the liver, which makes liver function tests necessary. The treatment, which lasts several months, is up to 80% effective. It can take from six to 18 months for complete results. And if you stop the medication too soon, the infection can rebound.

More recently, the use of lasers to treat toenail fungus has grown in popularity. This requires multiple treatments and, because data is somewhat limited, the efficacy is not yet clear. An even newer approach, known as cold atmospheric pressure plasma, is now being explored. Also referred to as CAPP, it involves the use of a device that delivers localized shock waves, which researchers say are effective at killing the fungus. This treatment is still being studied and is not yet in use. However, considering the millions of people affected by toenail fungus, it will likely make headlines if and when it becomes available.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)