The Surgeon General released a report in 2006 regarding the dangers of secondhand smoke (SHS). One of the claims in the report is:

There is NO risk-free level of secondhand smoke exposure, with even brief exposure adversely affecting the cardiovascular and respiratory system.

We know that secondhand smoke harms people's health, but many people assume that exposure to secondhand smoke in small doses does not do any significant damage to one's health. However, science has proven that there is NO risk-free level of exposure to secondhand smoke. Let me say that again: there is no safe level of exposure to secondhand smoke.

Breathing secondhand smoke for even a short time can damage cells and set the cancer process in motion. Brief exposure can have immediate harmful effects on blood and blood vessels, potentially increasing the risk of a heart attack. Secondhand smoke exposure can quickly irritate the lungs, or trigger an asthma attack. For some people, these rapid effects can be life-threatening. People who already have heart disease or respiratory conditions are at especially high risk.

This claim has appeared more than once within answers here on Skeptics.SE. Before taking on the more difficult question of does SHS cause lung cancer, it should be easy enough to determine if...

There is NO risk-free level of secondhand smoke exposure.


3 Answers 3


There are several different forms of dose-effect curve possible. A toxin following the "threshold model" has no adverse effect up to a particular threshold, and increasingly strong adverse effects past that threshold. "Hormesis" is a model under which small doses have a beneficial effect, but increasing dosage beyond a particular point has adverse effects (common amongst essential nutrients - for example, drinking moderate amounts of dihydrogen monoxide is beneficial, but in excess can cause water toxicity). Another, curve, the "linear no-threshold model", applies for substances whose risk factor increases linearly as the concentration rises, with no zero-effect threshold.

These different curves are illustrated in the diagram below, taken from a set of lecture notes on toxicology created by the International Union of Pure and Applied Chemistry.

various dose-response curves

The surgeon-general is talking about which dose-effect curve that second-hand smoke follows. For example:

This finding suggests that the effect of secondhand smoke in producing an MI comes from interference with the vascular endothelium. There is no evidence indicating a threshold level of exposure that is needed to produce this effect.

To say that "there is NO risk-free level of secondhand smoke exposure" is to say that "secondhand smoke follows a no-threshold toxicant model of dose-effect curve".

Many researchers argue that chemical carcinogens follow a no-threshold model, because it just takes a few molecules of a carcinogen can damage a cell and cause cancer [1], [2].

The report cites a number of studies which find that even low levels of exposure to secondhand smoke carries increased risks:

(not a comprehensive list, that was just from a couple tables that were convenient to skim)

Getting accurate numbers on this question is necessarily difficult - someone's exposure to secondhand smoke over a lifetime is a lot more variable than active smoking, even moreso when we're talking about low-level exposures rather than, say, someone married to a smoker. And low exposures are always going to be more difficult to separate from the controls - that's the nature of a dose-response curve.

However, it certainly seems that low exposures to secondhand smoke have been shown to have adverse impacts.

  • so when the SG repeats himself, "no safe level of SHS," you are saying that her intent was to inform people that SHS follows the no threshold (carcinogen) model and was not repeated as a scare tactic? It is curious that the SG doesn't warn us about other avoidable carcinogens in drinking water, BBQ, bananas, enclosed building environments, ... since those also have no safe levels. What you and the SG are saying is true, but misleading. When I finish my is SHS dangerous answer I will link it here and it will become more clear.
    – user1873
    Commented Jul 5, 2012 at 20:03
  • @user1873: The potassium in bananas is both required and under-consumed. It must follow the essential nutrient curve, so that explains that one. Distilled water isn't safe, so that explains why regular tap water doesn't get a warning. I think her position is sensible if people wrongly assume that tobacco smoke is a threshold toxicant & every day low doses are fairly toxic & lots of people are being exposed.
    – Oddthinking
    Commented Jul 6, 2012 at 4:56
  • @Oddthinking, you can drink distilled water that has had minerals added back into it. I think you mean SHS is a no-threshold toxicant. Do we know that no-threshold toxicants actually exist? "This hypothesized mechanism for carcinogenesis is referred to as "nonthreshold" because there is believed to be essentially no level of exposure to such a chemical that does not pose a finite probability, however small, of generating a carcinogenic response."
    – user1873
    Commented Jul 6, 2012 at 5:28

As a skeptic, any claim which is accompanied by the phrase, "the science is settled," or "science has proven," should set off alarm bells. You will also notice a peculiar high usage of the word "can" in explaining the possible consequences of brief secondhand smoke exposure. The truth of the matter is:

  • Every "safe" chemical has some level at which it is not safe.
  • Every "dangerous" chemical has some level at which it is safe.

The father of toxicology, Paracelsus said, "All things are poison, and nothing is without poison; only the dose permits something not to be poisonous."

A real world example might be in order:

Dihydrogen Monoxide (DHMO) is a colorless and odorless chemical compound, referred to by some as Hydrogen Hydroxide, Dihydrogen Oxide, or simply Hydric Acid. This chemical is used in the development of genetically engineered crops, and is used as an industrial solvent and coolant. Fruits and Vegetables once contaminated with DHMO cannot be decontaminate, even by carefully washing them. DHMO is found in many carbonated beverages, and isn't even listed as an ingredient in many supposedly "all-natural" fruit juices.

The acute toxicity of DHMO as measured by its median lethal dose (LDv50) is only 90g/kg, at which half the test subjects die. Babies are especially susceptible to DHMO because of their low body weight (avg. 3.5 kg). DHMO will pass to the unborn children of mothers, and is passed to babies through their mother's breast milk. In solid form, DHMO has been known to cause severe tissue damage.

The air isn't even DHMO free. DHMO was found to be a major component of acid rain. DHMO concentrations in the air can be greater than 30g/m3. Air quality concerns have resulted in some states canceling recess on days in which the air quality is poor, but DHMO is not a measured pollutant in the EPA's Air Quality Index, so it is impossible to know how safe the air is.

DHMO is the leading cause of unintentional injury death worldwide, and the highest rates are among children 1 to 4 years of age. From 2000 to 2009, the babies and newborn death rate increased from 23.1 to 27.7 per 100,000, this is greater than half the rate of all deaths due to Lung and Bronchus Cancer 52.2 per 100,000 = (158,592 total US deaths LC) /(304,059,724 Estimated US population) * 100,000. If you count deaths at all ages, DHMO was responsible for an estimated 3,880 deaths in the USA, more than the estimated deaths each year due to secondhand smoke (3,400 per year).

Even with all the known dangers of Dihydrogen Monoxide, many industries continue to lobby congress to limit regulation of DHMO use in agriculture and industry. The EPA has been powerless to stop big business from dumping DHMO into our rivers and streams, or from spewing it directly into the air we breathe. Environmental organizations like the Dihydrogen Monoxide Research Division have been actively spreading the truth about the dangers of DHMO, and extensive polling have shown the public supports a DHMO ban in the United States.

The above statements are all 100% true, but extremely misleading. If you are currently attempting to protect yourself from the dangers of Dihydrogen Monoxide, you will likely be issued a Darwin Award. This dangerous chemical is water.

All "dangerous" chemicals are also safe at some level. Many of the chemical compounds listed below are found in secondhand smoke. To the right of these chemicals, is the Maximum Contaminant Level (MCL) allowed by the EPA in drinking water with notes indicating possible natural causes.

|Arsenic       |0.010 mg/L3 |(Erosion of natural deposits)
|Lead          |0.015 mg/L3 |(Erosion of natural deposits)
|Benzene       |0.005 mg/L3 |
|Cadmium       |0.005 mg/L3 |Corrosion of galvanized pipes; erosion of natural deposits
|Chromium      |0.100 mg/L3 |Discharge from steel and pulp mills, erosion of natural deposits.
|Benzo[a]pyrene|0.002 mg/L3 |
|Toluene       |1.000 mg/L3 |Discharge from petroleum factories
|Vinyl chloride|0.002 mg/L3 |Leaching from PVC pipes;discharge from plastic factories

Lead naturally occurs in the soil and water. EPA limits Lead to 15 ppb (parts per billion). If your drinking water does not exceed that level, it is of no concern.

  1. Does my water have lead in it above EPA’s “action level” of 15 parts per billion (ppb)?

If the answer is no, your water does not contain lead at current levels of concern.

The EPA says we shouldn't be concerned about lead in our drinking water that is below an action level, but the Surgeon General wants to repeatedly warn us that NO risk free level of second-hand smoke exists. Lead is a carcinogen whether it is in your drinking water, or in your smoke. It seems foolish to warn us about exposure from one source while ignoring the other.

Radiation is a potent carcinogen that has a low recommended exposure limits each year (5 rem/year whole body OSHA, 100 mrem/year general public, but it is used as a treatment to cure Prostate Cancer. The benefits of radiation therapy almost always exceed the risk.

Damage occurring at the cellular or molecular level, can disrupt the control processes, permitting the uncontrolled growth of cells cancer This is why ionizing radiation's ability to break chemical bonds in atoms and molecules makes it such a potent carcinogen.

Your food isn't even safe. Bananas because of their high potassium content, contain more [radioactive Potassium] than other fruit. Eating 2 bananas a day for a year would expose you to more radiation than you would get from a single chest x-ray (about 10 mrem).

Chinese oil cooking fumes, barbequed foods causing Polycyclic Aromatic Hydrocarbons in the food or carcinogens from the wood smoke itself, there is no risk free exposure to anything.

There is NO risk-free level of blank exposure

Fill in the blank however you want, water, bananas, BBQ smoke, Chinese grills, or anything else. Living is full of risk, the only away to avoid all risk is to cease to exist. The Surgeon General's warning about there being no risk-free level of secondhand smoke exposure is misleading, and ignores hundreds of years of toxicology that show that all poisons have some minor exposure level that is not harmful. If brief exposure to secondhand smoke has you running scared, don't forget about thirdhand smoke.

  • I see a contradiction between "There is NO risk-free level of blank exposure" and "all poisons have some minor exposure level that is not harmful". I think the phrasing of the latter could be improved. Also, some sort of reference for your main claim that is more modern than Paracelsus would help.
    – Oddthinking
    Commented Jul 4, 2012 at 7:40
  • @Oddthinking, the EPA site I linked to mentions many of the same substances that are in cigarettes, and the Maximum Contaminant Level allowed in drinking water. As with the case with Lead, if you see levels that aren't higher than the MCL listed, they aren't of concern. The idea behind this I believe is that as Paracelsus stated, there is poison in everything (erosion natural deposits) and you would likely die of something else before the dose below the MCL would kill you. Might be a contradiction, but that comes down to if it is possible you where dealt harm if you died of something else?
    – user1873
    Commented Jul 4, 2012 at 8:02
  • 5
    "Every "dangerous" chemical has some level at which it is safe." <- This should be cited. It's a plausible story that a single molecular causes a single mutation and that mutation produces cancer. The key issue is whether the risk grows linearly with the amount of toxin exposure or whether it grows exponentially or sigmoid.
    – Christian
    Commented Jul 4, 2012 at 9:13
  • @Christian, I don't think it will be possible to find a citation for every chemical. While your scenario is plausible, I don't think that is the key issue. The SG is stating that SHS has no risk free exposure level. What is needed, is to show either that the chemicals in SHS occur naturally and are unavoidable (shows exposure to SHS isn't the cause, it is nature), or to show that all chemicals have no risk free exposure level (the LD50 value shows when half die, but with a big enough sample size we could find where 1 dies). Many of the common SHS chemicals are listed in drinking water.
    – user1873
    Commented Jul 4, 2012 at 14:06
  • 1
    @romkyns, Did you read my answer? In list Benzene, chromium, cadmium, lead, etc. that have allowable levels in drinking water, but are carcinogens with no safe dose in secondhand smoke. You are complaining that I should have used a better real world example, which is exactly what I did.
    – user1873
    Commented Jul 20, 2012 at 12:09

I still prefer my more humorous answer, but with the idea that it is better to keep it simple, I have added this answer as well. We will see which one people prefer.

The Surgeon Generals key claim revolves around the fact that 69 chemicals in second-hand smoke are carcinogens, so there is no risk free exposure level. From the EPA's Toxicology Assesment:


Carcinogenesis, unlike many noncarcinogenic health effects, is generally thought to be a phenomenon for which risk evaluation based on presumption of a threshold is inappropriate. For carcinogens, EPA assumes that a small number of molecular events can evoke changes in a single cell that can lead to uncontrolled cellular proliferation and eventually to a clinical state of disease. This hypothesized mechanism for carcinogenesis is referred to as "nonthreshold" because there is believed to be essentially no level of exposure to such a chemical that does not pose a finite probability, however small, of generating a carcinogenic response. That is, no dose is thought to be risk-free. Therefore, in evaluating cancer risks, an effect threshold cannot be estimated.

So while it is true that there is no risk free exposure to SHS, there is also no where on earth that you can be free of natural occurring carcinogens. Report of the United Nations Scientific Committee on the Effects of Atomic Radiation to the General Assembly

11 . Everyone is exposed to natural radiation. The natural sources of radiation are cosmic rays and naturally occurring radioactive substances existing in the Earth itself and inside the human body. A significant contribution to natural exposure of humans is due to radon gas, which emanates from the soil and may concentrate in dwellings.

Since radiation is everywhere, the Surgeon General could just as well had said,

There is NO risk free level of living on earth.

It would be just as true, and less misleading. Now onto the harder question of, "does secondhand smoke cause lung cancer, and is it dangerous."

  • 1
    But you can choose not to smoke or be around people who are smoking. so this is an avoidable risk. The question really is "Is there good reason to avoid that risk completely?" FWIW - I liked your more humorous answer better too.
    – Chad
    Commented Jul 5, 2012 at 14:20
  • 1
    @Chad, you can avoid bananas, and drinking water (distilled water), etc. It is foolish to say that there is NO risk free level of SHS, if you are unable to differentiate from deaths due to SHS, and deaths due to the noise of random background radiation. Perhaps there are unintended consequences of avoiding bananas, drinking water, and SHS that we are unaware of. Without knowing the risk of avoiding those things, you cannot judge the risk of exposure. The Surgeon General's NO risk blanket statement is misleading and useless.
    – user1873
    Commented Jul 5, 2012 at 14:47
  • 1
    I will give you a +1 for There is NO risk free level of living on earth. I think that sums it up nicely
    – Chad
    Commented Jul 5, 2012 at 14:50

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