Dentist Beware – Nitrous Oxide in Use!

Years ago, when OSHA was just getting started, an industrial hygienist friend of mine, told me “that if you are going to see a dentist, go early in the morning – if you wait, he might be groggy from inhaling nitrous oxide”.  Well, I would not expect to have the wrong tooth pulled if I waited until 3PM., but it is conceivable that his (and these days, or her) finer judgment or muscular control could be affected.  It may sound silly, but to this day I opt for earlier appointments if there is some possibility my doctor might be exposed to some anesthetic gas.  Fast forward:  I just read that a clinic has been fined $10,000 (later reduced to $1,800) for high employee exposure to nitrous oxide.  It is easy to understand why nitrous oxide can be such a persistent problem. Consider:

  • Nitrous oxide is customarily used in large volumes at a high concentration.. Usually, the mix is about 70% oxygen to 30% nitrous oxide.  This has the potential to release large quantizes of nitrous oxide into the clinical environment
  • Although highly concentrated nitrous oxide has a slightly sweetish odor, at low concentrations it has no perceptible odor.  Without an instrument, you cannot tell that an unacceptable concentration is present. There are, fortunately good tools to monitor the exposure to nitrous oxide.  For finding leaks, a portable device such as an infra-red meter may be used.  To measure personal exposure, diffusive samplers are commonly employed.
  • Despite best efforts, ventilation systems can fail. Especially if the wind is blowing in such a direction that the exhaust air is drawn back into the clinic.  Years ago I used the technique of releasing a bolus of nitrous oxide into the room of a clinic and following its subsequent removal via ventilation.   I was often surprised by how persistent the nitrous oxide remained in the room air.  To paraphrase an old saying, if you work in a dental clinic, “Breather Beware”.
  • ·       There is excellent information on the control of nitrous oxide in the dental clinic, involving both engineering design and proper use of equipment,   OSHA expects that the clinical use of nitrous oxide is consistent with such guidelines.[i],[ii]
  • There is no OSHA standard for nitrous oxide.  OSHA has a horrid problem in setting new standards – one estimate is that if the full process for setting a standard is followed, given the time required for hearings, appeals, publishing in the Federal Register, etc. it takes about eighteen years for new standard to take effect.  Instead OSHA relies on the “General duty clause”, which often means that the standard, if not already established by OSHA, relies on other respected standards, such as the TLV, which is an exposure level established by the American Conference of Governmental Hygienists.  Their TLV for nitrous oxide is 50 ppm (90 mg/m(3)) as a TWA for a normal 8-hour workday and a 40-hour workweek

Point of disclosure: Twisuk Punpeng, before he became my doctoral student, made fundamental findings about the adsorption of nitrous oxide on the type of molecular sieve now used by Sensors Safety and other companies to sample nitrous oxide.[iii]  I’m continually surprised by the interconnections in this small, small world..

I should have known better (A true story)

Twenty five years ago I was a newly minted CIH, with a bride that I absolutely adored.  And I still do.  But when we moved into our home, my wife began to experience nose and throat irritation to such an extent that it really became depressing for us both.  I was completely lost as to the cause.  After a few months, the symptoms gradually disappeared.   Marilyn never gave up on finding the cause of her symptoms.  Several years later she read up on the health effects of formaldehyde exposure, and found she had undergone a close pattern to a number of the symptoms listed below.  She is convinced that the cause was the new carpeting we had installed just before moving in to our home.  So am I.  If we had only realized this at the time, and tested for indoor formaldehyde, something easily done,  so much very real suffering would have been ended at once, as we would have had the offending carpeting taken out immediately.  Better to test than be sorry!  To understand better what we faced, the table below is a list of formaldehyde-related symptoms compiled by the U.S. Consumer Product Safety Commission.




Potential adverse health effects
Eyes • Stinging, burning, or itching

• Excessive tearing

Nose or throat • Stinging, burning, or itching

• Sore throat

• Runny nose

• Blocked sinuses

• Sneezing

• Cancer (human and laboratory animals)

Respiratory • Chest tightness

• Wheezing

• Asthma

Skin • Allergic contact dermatitis

o Skin rashes, blisters, and flaky dry skin

Neurological • Headaches

• Mood changes (i.e., depression, irritability)

• Insomnia

• Attention deficit

• Nausea

• Impairments in dexterity, memory, and equilibrium



Information Overload!

What’s a Health Administrator to do?  We at Sensors Safety try to give you the clearest possible answers, but there is so much (perhaps too much) information out there that good decision making may become difficult.  For example, to see how difficult this can be, skim over the following information from a real Material Safety Data Sheet, and decide how you would handle the following dangerous chemical in your laboratory.  If you do read through the following, I promise you will get a real surprise at the end.


Potential Health Effects 
Eye: May cause eye irritation. 
Skin: May cause skin irritation. 
Ingestion: Ingestion of large amounts may cause gastrointestinal irritation. Ingestion of large amounts may cause nausea and vomiting, rigidity or convulsions. Continued exposure can produce coma, dehydration, and internal organ congestion. 
Inhalation: May cause respiratory tract irritation. 
Chronic: No information found.

Section 4 – First   Aid Measures

Eyes: Flush eyes with plenty of water for at least 15 minutes, occasionally lifting the upper and lower eyelids. Get medical aid. 
Skin: Flush skin with plenty of soap and water for at least 15 minutes while removing contaminated clothing and shoes. Get medical aid if irritation develops or persists. Wash clothing before reuse. 
Ingestion: If victim is conscious and alert, give 2-4 cupfuls of milk or water. Get medical aid. Wash mouth out with water. 
Inhalation: Remove from exposure to fresh air immediately. If not breathing, give artificial respiration. If breathing is difficult, give oxygen. Get medical aid if cough or other symptoms appear. 
Notes to Physician: None

Section 5 – Fire   Fighting Measures

General Information: Water runoff can cause environmental damage. Dike and collect water used to fight fire. Wear appropriate protective clothing to prevent contact with skin and eyes. Wear a self-contained breathing apparatus (SCBA) to prevent contact with thermal decomposition products. Substance is noncombustible. 
Extinguishing Media: For small fires, use water spray, dry chemical, carbon dioxide or chemical foam.

Section 6 –   Accidental Release Measures

General Information: Use proper personal protective equipment as indicated in Section 8. 
Spills/Leaks: Vacuum or sweep up material and place into a suitable disposal container. Clean up spills immediately, observing precautions in the Protective Equipment section. Avoid generating dusty conditions. Provide ventilation.

Section 7 –   Handling and Storage

Handling: Use with adequate ventilation. Minimize dust generation and accumulation. Avoid contact with eyes, skin, and clothing. Keep container tightly closed. Do not ingest or inhale. 
Storage: Store in a cool, dry, well-ventilated area away from incompatible substances. Store protected from moisture.

The surprise: This is a MSDS for table salt!  This type of information given for something you sprinkle on your food can divert your efforts from protecting employees from really dangerous chemical hazards. A more realistic MSDS for the same substance is given at:  As an aside, and I can vouch for this personally: At Sensors Safety we make every effort possible to present the information you need in a truly informative manner.

Dr. Dwight Underhill, CIH



An Informal History of Diffusive Sampling

Believe it or not, diffusive sampling in the workplace goes back at least to the very early 1900’s.   At that time it was a practice to use filter paper moistened with a solution of some compound that would change color in the presence of a harmful gas or vapor.  For example, a solution of silver or lead nitrate would turn black in the presence of hydrogen sulfide, a solution of phenolphthalein and very dilute lye would change from bright pink to colorless in the presence of dangerous concentrations of carbon dioxide.  Other solutions were formulated for phosphene and for oxides of nitrogen.  The results were far from quantitative, but they did achieve the basic goal of worker protection.

The next major step in diffusive sampling was developed by the CIA!  It was called a sneaky, and consisted of the following.  A charcoal impregnated piece of cloth was placed between two sheets of white, highly opaque cloth, then cut and sewn to appear no different from an ordinary handkerchief.  This “handkerchief” would be placed in a cigar tube, and given to someone, a diplomat, etc., visiting an interesting place, for example a Soviet factory.  The “diplomat” would pull out his “handkerchief”, and expose it to the ambient air.  Vapors from the factory or whatever would drift into the handkerchief and be captured by the charcoal.  When the visit was finished, the “handkerchief” was returned to its cylinder, later taken back to Langley, and analyzed by gas chromatography for whatever chemicals were present.  Helping this process was the fact that Soviet industrial hygiene was so poor that there were, I’m certain, often ample material to analyze.

The trouble with these detectors was that they depended both on the concentration of contaminant and the wind speed, making it impossible to precisely determine the concentration of contaminant.  The really big step forward was by Ed Palmes, who devised a diffusive sampler in which diffusion, not wind speed, determined the uptake.  There samplers were originally devised for sampling indoor nitrogen dioxide, and are still used for that purpose.

A typical commercial Palmes tube is made of commercial acrylic tubing, outside diameter 0.5 inches, inside diameter 0.38 inches, and 2.8 inches long.  It is sealed on one end and on the other end there is an easily removable cap, which is removed during sampling.  Held at the bottom of the tube are (usually three, 40 X40 mesh) stainless steel screens coated with triethanolamine, if the sampling is for nitrogen dioxide.  At the end of the sampling period, the tube is sent to the laboratory for analysis of absorbed nitrogen dioxide.

Others developed related sampling devices.  Some were shaped more like Sensor’s badges, but instead of needing a laboratory to make an analysis, there is a color change, the extent of which depends on the concentration of contaminant.  The defect in such a sampler, at least for OSHA compliance, lies not in the sampler, but rather in you and me!  The problem is that the human eye cannot easily distinguish differences in color as small as 25%, but that is too much error to allow.  Instruments such as gas chromatographs can detect such small differences, and this is what makes diffusive sampling a viable technique.   I think that the real “coming of age ”  for diffusive samplers began a few years back when OSHA started to use them in their surveys.  This meant that OSHA believed –just as we do – that the evidence taken by a diffusive sampler can stand up in court before a jury, even following hard cross examination by a skilled attorney.  My, how so much has changed in a hundred years!

Dr. Dwight Underhill, CIH


Benzene Toxicity

Benzene is one of the primary analytes in our proficiency tests and a chemical we regularly test for.  This brings to mind that some years ago – actually it was many years ago – when I was a graduate student at the Harvard School of Public Health, one of our laboratory experiments consisted of exposing the students (including me) to benzene vapor in a chamber.  The next day we each brought in a sample of urine to be analyzed for evidence of benzene exposure.  Here is how the test worked.  In the human body, a considerable fraction  of the inhaled benzene is oxidized to phenol.   Then a limited fraction of this phenol is conjugated to become phenyl sulfate.



The actual analysis consisted of measuring the ratio of phenol in the urine to the amount of phenyl sulfate in the urine[i]. Because the amount of conjugation possible over a given time period is limited, if the ratio of phenol to phenyl sulfate is very high, this indicates that the body was exposed to so much benzene that it could not adequately detoxify the phenol (which is also a very toxic compound) by adding a sulfate group to it.  As I recall, the day I brought in my urine sample was the first and last time I ever tested positive for anything.

Now fast forward to the present.  If I and several of my classmates came down with leukemia, which is the cancer of major concern with benzene exposure, should we file (you can see this coming) a class action suit against Harvard for injury?  Read on to see my answer.

The toxicity of benzene can be divided roughly into three primary categories, as follows:



Exposure   Concentration

Exposure Duration

Acute   Poisoning

Hundreds of   PPMs


Bone marrow   damage

Tens of PPMs



??? PPMs



Acute poisoning usually happens in an enclosed space such as a tank or vessel with benzene residues, or from spills or equipment failure. Acute poisoning affects the central nervous system with symptoms such as dizziness, excitement, staggering gait; also headache, nausea, fatigue, insomnia, flushed face, incoherent speech, tingling in hands and feet. Symptoms can last up to two weeks and the length of recovery will depend on the severity of exposure. However, if the exposure is severe enough, the breathing center of the brain is paralyzed and death occurs[ii].

Symptoms of bone marrow damage are vague and thus deceptive. Tiredness, dizziness, headache, nausea, loss of appetite, weight loss and general weakness can easily be attributed to other causes. It is only later in the course of the disease that nosebleeds, bleeding gums, pallor and purple disfigurations appear[iii].

The leukemia associated with benzene is thought to arise from the bone marrow damage described above.  As my exposure could not have given me bone marrow damage, then if I ever came down with leukemia, it would not be a likely consequence of my one brief benzene exposure.  A judge in a court of law, hearing my law suit should say “Case dismissed”.  As an afterword, the correlation between benzene exposure and leukemia was just becoming known at the time of my chamber exposure.  I just finished looking up references to this via the library at the National Institute of Medicine (using and found, dating to the time of my brief exposure, three references to leukemia from benzene exposure, of which two were from foreign authors, the other in a state medical journal.  And I must say, all the professors I met at Harvard truly only wanted the best for their students, and would be hurt beyond belief if any of their students were actually harmed by something they did.

Dr. Dwight Underhill, CIH


Want to know more about Perchloroethylene?


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The Agency for Toxic Substances and Disease Registry (ASTDR) is so concerned about Perchloroethylene (tetrachloroethylene, better known as Perc) that it developed an online course – their Course Number WB 1110 – for physicians who need to know more about the … Continue reading

Formaldehyde Exposure: How should it be sampled and by what standards?

How should you sample for formaldehyde exposure, and what standard should you use?  The sampling procedure depends on the type of source.  In a factory producing resins imparting wrinkle-resistance to clothing (a major use of formaldehyde), one might get a satisfactory result by area sampling.  On the other hand, if one were to determine the exposure of a hair dresser to a spray possibly containing formaldehyde, sampling in the breathing zone is essential, as the formaldehyde concentration will decrease dramatically with distance from this point source. 

What exposure standard should you use?  The OSHA permissible exposure limit (PEL) for formaldehyde in all workplaces covered by the OSHA Act is 0.75 ppm measured as an 8-hour time weighted average (TWA). The standard includes a 2 ppm short-term exposure limit (STEL) (i.e., maximum exposure allowed during a 15-minute period). The “action level” is 0.5 ppm measured over 8 hours.[v][5]  The corresponding NIOSH recommended exposure standard is a TWA of 0.016 ppm with a 15 minute ceiling of 0.1 ppm.  Only the OSHA standard is federally enforced, but the NIOSH standards, which for the TWA actually forty eight times lower than the OSHA standard, implies that if even if OSHA standards are met, if there are reasonable ways to reduce the exposure to formaldehyde, they should be implemented. 

My additional thought on this, for what it is worth, is that the NIOSH levels are unrealistic.  Sax et al. and Weisel et al. monitored homes across the U.S. that are representative of the housing stock and reported median residential indoor concentrations of formaldehyde that are between 1.9 and 2.4 times higher than the NIOSH recommended exposure limits.[vi][6]   The incongruity is even greater when it is remembered that home exposure can be nearly 24 hours a day, seven days a week; whereas the NIOSH exposure levels are for 8 hours a day, five days a week. 

Points of disclosure:  Formaldehyde and I have been “friended” for some time.  I supported myself in graduate school by setting up formaldehyde concentrations for animal inhalation experiments.  My first chemistry set (I was preteen then) had a test for formaldehyde in milk.  My father had to help me with this because it involved heating milk in an acid solution. 

Dr. Dwight Underhill, CIH


[vi][6] Formaldehyde in residences: long-term indoor concentrations and influencing factors D. E. Hun et al. Indoor Air 2010; 20: 196–203


Danger: Formaldehyde Exposure!

The effects of formaldehyde exposure in humans are well known, and include:

  • Inhalation of formaldehyde can give irritation and burning of the mucous membranes of the nose, mouth and upper respiratory tract
  • Severe inhalation may cause weakness, headache, nausea, vomiting, pneumonia, dyspnea, wheezing, coughing, laryngeal and pulmonary edema, bronchospasm, respiratory depression, laryngeal spasm, CNS depression, convulsions and coma.  Following an acute inhalation exposure, the onset of pulmonary edema may be delayed for 24 to 48 hours.
  • Acute ingestion will cause irritation, ulceration, burns and hemorrhage to the gastrointestinal tract, as well as metabolic acidosis, tachypnea, jaundice and acute renal failure. Ingestion of 50 grams of formaldehyde has a good chance of being fatal.
  • Formaldehyde is corrosive and can cause irritation and burns to the skin and irritation of the eyes. Ocular exposure may result in permanent vision alterations or blindness[i][1]

Formaldehyde is also an allergen.  To understand why, remember that it binds to protein molecules.  If the body perceives these altered proteins as “foreign”, it can mount an attack against them, resulting in what we see as an allergenic reaction.  Thus some people develop a severe dermatitis on skin exposure to formaldehyde.  If you suspect that someone is having such a reaction, a dermatologist can apply a skin test using formaldehyde to find if there is contact dermatitis due to formaldehyde exposure.  There is an interesting historical example of such a reaction.  Before World War Two, Revlon briefly sold a nail hardener containing a rather high concentration of formaldehyde.  A few women developed acute reactions to it.  To their credit, when Revlon learned of this, they did the right thing.  One of the Revlon brothers went out to make certain that all samples were recalled.[ii][2]

Despite the vast literature on the subject, according to OSHA there is some controversy regarding whether formaldehyde gas is a pulmonary sensitizer which can cause occupational asthma in a previously normal individual.[iii][3]

Formaldehyde is commonly thought to be a carcinogen.  At high concentrations it causes nasal cancer in rodents.  The same effect is, apparently seen in humans through epidemiologic studies in workers exposed to high concentrations of formaldehyde, but this effect is very difficult to demonstrate.  The first human studies to find human nasal cancer associated with formaldehyde exposure were negative, and to find a positive effect, the results from a number of studies had to be lumped together.  Recently there has been reported a causal association with myeloid leukemia.  Thus formaldehyde considered is to be a human carcinogen, but it is definitely not a strong carcinogen, for if it were OSHA would have banned its use in the workplace, just as it did with a number of strong carcinogens such as bischloromethyl ether.  On the other hand the mechanism by which formaldehyde could cause cancer is well understood.  It binds to DNA, causing an error when the strand holding the attached formaldehyde tries to replicate.

This may be surprising, but in my opinion, formaldehyde is likely not important as a human carcinogen.  A closer look at the animal experiments bears this out. According to Health Canada (the Canadian equivalent to OSHA): Carcinogenicity studies consistently found an increased incidence of carcinomas of the nasal cavity at levels of 6.7 mg/m3or over; no such tumors were found at lower concentrations (up to 2.4 mg/m3). Formaldehyde-induced carcinogenicity appears to be a consequence of proliferative regeneration following cytotoxicity. The risk of cancer associated with formaldehyde levels sufficiently low to prevent irritation and inflammatory responses appears therefore to be negligible. .[iv][4]

The same effect occurred in the Canadian studies of the carcinogenic effect of saccharine.  The male rats in this study developed urinary tract cancers.  Saccharine was declared a carcinogen and nearly banned.  But female rats didn’t get cancer.  Reason: The rats were given the equivalent of 500 sodas a day.  Crystals of saccharine were forming in the male urinary tract and their irritation was causing cancer.  Thus if you are male and drink 500 sodas a day, you might get cancer from saccharine, otherwise you are safe.  A similar phenomenon may be occurring with formaldehyde.  I have a great difficulty in believing meta-analyses in which studies are combined, because there is so much at stake in how the studies are selected and combined.  For example journals are loath to publish studies that show a negative effect or no effect at all.  If you select data from only studies hinting at a positive effect, your meta-analysis will show a strong positive effect, when in fact there is none.

Although the carcinogenicity of formaldehyde may remain somewhat in question, its other toxic effects are not.  Workers should not be overexposed to this chemical

Dr. Dwight Underhill, CIH

Get Your Reports Online

From time to time I will get a frantic customer on the phone needing 3 years of their department’s reports faxed to them ASAP because they have auditors at their facility that day.  Of course we are happy to supply them with what they need, but I hate that they have to go through all that worry looking for the documentation required of them.

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Why Is Formaldehyde Found in So Many Products?

Recently I came across an article warning of significant formaldehyde exposures in beauty salons.  Add this to formaldehyde’s presence in histology labs, sterilization facilities, fingernail hardeners, embalming fluid, plastics, glues, etc., and you might ask: What is it doing in all these places?  The answer lies in the fact that formaldehyde can cross bind.  This means that this small molecule (formula HCHO) can actually form two separate bonds to organic compounds.  Therefore binding them together in an unnatural three dimensional web, renders them rigid and nonfunctional.  Applied in a hair product, formaldehyde can give the hair a “body” that it’s never had before.  Applied to fingernails, the nails harden nicely.  In a histology lab, it can fix tissues so that they can be sliced into very thin sections.  Applied to a pathogen, all its protein molecules are immobilized, and it is dead!
Dr. Dwight Underhill, CIH