"Dr, your staff was exposed to excessive levels of nitrous oxide; you are fined $10,000."  That is exactly what happened to a dentist in Washington State.   I was asked by the staff of the Washington State Dental Association to look into this case.  It was the third case of dental offices being cited by the State that I had reviewed.  This was the first time there had been a fine.  In fairness to the State, it should be noted that the fine was dropped to $1800 on appeal.  On the other hand, there is no science to show that anyone was ever at risk at the levels reported.  It is also very doubtful that the levels reported by the state were ever reached.

When I reviewed this case, I found many problems with how the inspection had been run.  An industrial hygienist from the state of Washington Department of Labor and Industries came to the office and used two monitoring techniques.  The first used a personal area sampler.  This device continuously samples air that is sucked into the machine via a hand held probe.  It reads on a minute to minute bases the levels of nitrous oxide in the area at the tip of the probe.  It is relatively portable so it can be moved around a room to find the most concentration.  Because it reads out in real time, it can be very effective in finding leaks or determining where pockets of nitrous oxide might be.  In this case, the industrial hygienist, stated in a debriefing report, "I did not get right (in the area) at the time, I felt it was imposing to try to get in there.  If you are using nitrous patients are apprehensive to begin with, so I you get another body in there with a Geiger counter saying "Oh my God itís off the scale" Spot rating that I got ranged anywhere form 19 up to about 25 ppm.  ... All indications that I had from that at the time is that I didnít expect exposures of this magnitude..."

Ten staff members were also required to wear a Landauer nitrous monitors.  These were worn for a day.  The monitors are than sealed and returned to the Landaure company and are evaluated to determine what the time weighted average (TWA) was for each person.  The results came back  3 to 1421 ppm.  Three monitors came back below 50 ppm, two between 50 and 100 ppm, two between 100 and 150 ppm, one each at 378, and 417 ppm and one at 1421 ppm.  It is difficult to explain the range of reading and the complete lack of correlation with the reading of the personal area sampler.

One possible explanation is that a decimal point was misplaced in the 1421 ppm reading.  The doctor in this case had readings between 11 and 17 times higher than the assistants who worked with him.  It is very hard to explain how his levels could be 10 plus times higher than the dental assistant sitting less than 3 feet away.  It is even more difficult to accept once you realize he moved between three rooms where the 3 assistants worked.  The time in the hall would help dilute the levels he was exposed to.  However if the lab read a 47.0 minute exposure vs. a 470 minute exposure, about 8 hours, this badge would become 1421 vs. 142.1 ppm.  Rather than being 10 times greater then the assistants, the three operators, two hygienists and a dentist who worked without scavenging would now be with in 50 ppm of each other.

This clinic was not without fault.  The rooms with the lowest levels of nitrous had scavenging devices.  The operatories with the higher reading did not. The two readings near 400 ppm were for dental hygienist that both worked in a room without scavenging for the whole day. The ventilation system of the office changed air about 3-4 times an hour as opposed to a suggested minimum of 5 changes per hour and a recommended 15 to 20 changes an hour.

The Washington Industrial Safety and Health Administration (WISHA) is part of the Department of Labor and Industries and is the States agency responsible for worker safety and must have regulations at least as stringent as the federal Occupational Safety and Health Administration (OSHA).  It should be emphasized that OSHA has no regulations.  In a WISHA debriefing of this case it was stated, "We ought to get in touch with our toxicologist to say most industrial applications you get an exposure of the type youíre going to see some real clear clinical symptoms.  ...The most immediate things you are going to see with nitrous is that there is some link with increased carcinogenicity, spontaneous abortion... Nitrous oxide has a considerably low permissible exposure limit by law in view of the toxicological effects."

Because the State of Washington has a regulation that puts the permissible exposure limits on a time weighted average at 50 ppm. the State fined this clinic.  The clinic appealed on the basis that they were not aware of the requirement of scavenging and immediately had installed scavenging in the rooms that did not have it after the inspection.  The appeal was denied.

The question that needs an answer, Was this clinicís levels of nitrous oxide hazardous? Was the state justified in its fine and statements that were made ?  At what level does nitrous oxide contamination become a hazard for the dental staff?  Is 50 ppm. on a time weighted average. an appropriate requirement?

My interest in the issue started with the first case where a dentist was cited by the state for have 35 parts per million (ppm.) permissible exposure limit for nitrous oxide in a dental office.  As it turned out, one of the lessors of his building want to break a lease and cited levels of nitrous oxide as the reason. He was cited by the state for a 35 ppm level. I contacted the state and was told the 30 ppm was a typographic error and they would correct it the next time they looked at PELs.  This dentist was forced through a costly legal battle because of the Stateís topographic error.  Multiple letters and 8 years passed but the change was made from 30 ppm. to 50 ppm.  After reviewing over 200 research abstracts I have come to the conclusion that there is not science to the 50 ppm. limit.

Historical overview of Nitrous usage.
One of the major challenges of dentistry is fear and pain control.  The two are closely related.  Fear of future treatment is often the result of lack of pain control.  Pain control is most often achieved with local anesthesia. Weinstein reported patients report the incidence of failure of the local anesthetic injection to be as high as 26.4%, 13% reported not being numb for drilling, 2.2% reporting they were unable to tolerate the treatment.  They suggested the fear had a high correlation with those who had anesthetic problems.

About half the US population avoids yearly dental care.  Between 6 and 14% avoid any treatment because of fear.   This phenomenon is not unique to the US.  Others have shown similar problems in Sweden and Japan    In an attempt to help patients with their fear many practitioners have patients breath a mixture of nitrous oxide and oxygen.  It is estimated about 50% of dentists have the equipment to administer this mix and more than 424,000 dental personnel are exposed to the trace amounts of gas as a result of its administration.

History of Nitrous Oxide Use
Nitrous oxide was discovered by Joseph Priestley in 1776.  Humphry Davey had written of the effect of nitrous oxide in the diminution of his pain from a wisdom tooth.  In 1840ís Gardner Quincy Colton, a chemist and intineratant lecturer introduced it to the public after reading of Daveyís book, Researches, Chemical and Philosophical;Chiefly Concerning Nitrous Oxide.  Colton introduced nitrous oxide to Horace Wells, a dentist, who became the first to use the gas for the relief of pain.  On December 11, 1844 Wells had a local dentist, Dr. John Riggs extract one of Wells teeth while under the effects of nitrous oxide.   The use of nitrous oxide in dentistry continued as a general anesthetic until the advent of local anesthesia.  With local anesthesia, dentistry had another way to control pain.  Dentistry divided into two factions those that used it as a general anesthetic and those who used local anesthesia.  Nitrous oxide surfaced several times in the early 1900ís as a sedative/analgesic often associated with nausea and vomiting.  It was in the 1950ís that Langa and others popularized its use as an antianxiety agent by showing it could be used as a sedative in conjunction with local anesthesia. If percentages were limited to 25-40%, few side effects were seen.

Occupational Safety
Occupational exposure to some chemicals, materials or ionizing radiation have been shown to cause serious health concerns.  One of the first cases of occupational hazard was scrotal cancer that was reported to be endemic in English Chimney Sweeps.  Liver disease is a toxic effect of exposure to carbon tetrachloride that was used in the dry-cleaning industry. Benzedrine has caused bladder cancer and x-rays have led to a variety of problems including cancer and spontaneous abortions.

Anesthesia Gas Problems
The first indication that anesthetic gasses might be a problem for humans was a report in 1967 by Vaisman who reported Russian female anesthesiologists had problems.  Problems included irritability, headache, fatigue nausea puritis, and 18 of 31 pregnancies ended in spontaneous abortion  Similar studies by Askrog, Harvard and Lencz and Nemes indicate that women who worked in western operating rooms faced similar problems.  It In1956 Lassen noticed that nitrous oxide appeared to inhibit the cell growth in human bone marrow when patient were left on 50% nitrous oxide for 14 days.  The marrow returned to normal within a few days once the treatment was stopped  was clear the OR had the potential to be a hazardous place to work but it was not clear which chemicals were the causative agents.

Animal Studies
Potent anesthetic agents had been shown to have teratogenic effects in animal studies. They were very potent at very low levels.  However, nitrous oxide was part of almost every anesthetic and also needed to be evaluated.  Eger showed a causal role for teratogenicity and miscarriage for rates that were exposed for 24 hours between day six and day 12 of pregnancy, increased fetal wastage on days 8 and 11 but not on other days.  He showed skeletal abnormalities on days 8 and 9.  However, he stated that causal role of these inhaled agents appears very low or nonexistent.   Abdul-Kareem exposed rats to nitrous oxide at 50, 500, 5000 ppm for 6 hr a day 5 days a week for 2 and 13 weeks and showed changes in neurotransmitters.  The 50 ppm. exposure only increased dopamine and serotonin.   In a similar study Healy exposed mice to 50, 502 or 5990 ppm for 5 days a week for 2 or 13 weeks.  Liver weight was lower for all three at 13 weeks but not a 2 weeks.  Leukocytes were lower at the highest level at 2 weeks and at all levels at 13 weeks.  The bone marrow showed a dose related decrease in deoxyuridine uptake.  Fujinaga showed problems with rat fetal mortality at 75% nitrous oxide.   Viera showed spontaneous abortions in rats at 1000 ppm.   Kripke showed testicular damage after a minimum of 2 days exposure to 20% nitrous oxide.   Green showed the bone marrow of rats was depressed with 80%  nitrous oxide but not 1% even after 6 months.   Nitrous oxide at 1000 ppm decreases the vitamin B12 (cobalamin or methylcobalamine) component of the enzyme methionine synthetase by interfering with the folate metabolism this can impair DNA synthesis.  In a study to simulate operating room conditions rats were exposed to 1ppm halothane, 50 ppm nitrous oxide or 10 ppm halothane and 500 ppm nitrous for 7 hours a day, 5 days a week for 104 weeks no increase in tumor incidences could be found.     Several studies showed increased fetal mortality or smaller litter size when rats were exposed to 1,000 ppm nitrous oxide if the rats were left on this concentration for a number of days, but no effect at lower concentrations.   Some studies showed no teratogenic effect for rats or mice exposed to 0.5, 5, or 50% for eight hours a day throughout gestation.   The animal studies suggest there is a problem associated with exposure to nitrous oxide.  They also hint that if the levels are kept low enough the problems can be lessened.

Retroactive Human Studies
The dental office was an ideal study site as there were two sets of offices, those that used nitrous oxide and those that did not. Cowen did two such studies in conjunction with the American Dental Association.  These studies suggest what the animal studies show.  There is a problem with higher levels of exposure.  These studies showed a higher incidence of hepatic, renal and neuralgic disorders among exposed personnel.  More importantly it showed an increase in spontaneous abortions in charside assistants and the wives of dentists.  It also showed an increase in congenital abnormalities in children of assistants.    There are several problems with these studies.  There is concern that respondents might not be accurate in recall of events in the last decade.  At the time there was public concern about the safety of anesthetic gas that may have biased the responses.  18.7% of the respondents were also exposed to other anesthetic gases.  No attempt was made to remove recreational users who abused nitrous oxide.   Although there may be some problems with these studies they did point to a problem for females who were pregnant and working in dental offices where nitrous oxide was used.  It did not suggest at which level exposure became a problem.

Occupational Hygiene Agencies
There are three governmental bodies associated with setting appropriate levels OSHA, NIOSH, ACGIH.  ACGIH, the American Conference of Governmental Industrial Hygienists publishes a set of suggested maximum exposure levels for many chemicals.  They make it very clear in their literature that these levels are only suggestions and guidelines to be used as a guide by industrial hygienists and should not be used to set regulations.  OSHA, the federal Occupational Safety and Health Administration, sets minimum guidelines for employers to assure a safe workplace for US workers.  Individual states may have their own departments, CalOSHA and WISHA, and regulations.  The state regulations must be at least as stringent as the federal regulations.  NIOSH, the National Institute for Occupational Safety and Health, is an occupational safety and health research arm of the federal Centers for Disease Control which makes recommendations to OSHA.   The NIOSH publication Alert suggests the recommended exposure limit (REL) of 25 parts per million (ppm) on a time-weighted average (TWA) of 25 ppm.  These levels were determined after reviewing two studies by Bruce.

The Problem Studies
The TWA level of 25 ppm came from two studies done by Bruce, Bach and Arbit. In the first study, a difference was shown when subjects were exposed to 50 ppm nitrous oxide with 1 ppm halothane but not to 500 ppm nitrous oxide except for a digit span test.  The differences were seen in audiovisual tasks, tachistoscopic tasks, memory passages and digit span tests.    The second study showed a slight effect to subjects exposed to 500 ppm nitrous oxide for 4 hours that were asked to recall a series of numbers both forward and backward, Digit span test, and a boarder line, .10>p>.05 difference on a paired associates test where subjects were asked to say the second member of a pare of words when the first one was given.   It is important to note that Bruce recanted this study as flawed in two letters one in 1983 the other in 1991.  He stated, There is no longer any need to refer to our conclusions as ëcontroversial.í They were wrong, derived from data subject to inadvertent sampling bias and not applicable to the general population.  The NIOSH standards should be revised."
Within weeks of these experiments and before they were published, representatives of NIOSH met with investigators working in the field of trace anesthetic exposure and decided to recommend routine scavenging in anesthetizing locations.  The question was asked, to what level?  This was answered: below the lowest level for which there is any evidence of ill effect and to which it would be technically possible to scavenge by simple, inexpensive means.  Our results at 50/1 (50 ppm nitrous oxide/1 ppm halothane) ppm were the only data at low anesthetic levels that indicated adverse human effects.  Since Charles Whitcherís studies a Stanford had shown it was possible to scavenge to 25 ppm. N2O, that was where the standard was set.  The 25 ppm N2O to 2 ppm halogenated agent ratio was an attainable standard for which no evidence of toxicity of any sort had ever been shown and was therefore agreed upon.
Several years later, we learned that most of the subjects we studied were Mormons, and as such might have been abnormally sensitive to depressant drugs in a manner similar to that of Stanleyís patients.í There is no longer any need to refer to our conclusions as ëcontroversial.í They were wrong, derived from data subject to inadvertent sampling bias and not applicable to the general populations.  The NIOSH standards should be revised"      It is important to notice the first the effect seen was at fifty parts per million of nitrous oxide combined with 1 part per million halothane, a very potent general anesthetic agent.  The lowest level in these two studies that an effect could be seen with nitrous oxide as a single agent was at 500 ppm and that was a borderline statistical finding, 0.10>p>0.05.  The first study was titled Anesthetic Effects on Perceptual, Cognitive and Motor Skills.  Many papers have been published in the dental literature which mention the motor skill effect which was not shown for just nitrous oxide in either study.  We do not want dentists practicing with decreased motor skills.  While this effect is often mentioned in dental literature.  Neither study showed this effect.  Others repeated these studies and were unable to show the effects.  The author repeated the study and was unable to show the effect and wrote two retraction letters that have been ignored by dental literature and by NIOSH and OSHA in their publications.  There has never been a study that showed a detrimental effect at 50 ppm of nitrous oxide.  One study showed a questionable effect at 500 ppm nitrous oxide and it has been retracted by the author because neither he or others have been able to replicate the effect.  These two studies and the publications of NIOSH that arose from these studies have been referenced in so many dental journals they have nearly become fact ignoring the two retraction letters.  Once a paper is published it is very nearly impossible to retract the paper.

What is a safe level of exposure?
What studies have been done with humans that suggest what levels personnel can safely exposed to?  Ahlborg showed Swedish midwives exposed to nitrous oxide and shift work show no difficulty getting pregnant unless they used nitrous oxide 30 or more times a month to assist with deliveries.   In another study of midwives he showed no increase in spontaneous abortions with exposure to nitrous oxide but with night shifts and high work loads and no nitrous he saw an increase.   Sweeney preformed a study on 20 practicing dentists.  In this study they practiced while wearing a nitrous oxide monitor at the conclusion of the study these dentist has bone marrow samples taken and analyzed for deoxyuridine suppression.  The exposures ranged from 50 ppm to over 5,000 ppm. on a time-weighted average.  All but 3 were above 400 ppm.    The only depression seen was in three dentists over 1,800 ppm.  Some showed no depression at over 5,000 ppm.  To set levels that would assure safety, Sweeney suggested we should not exceed 450 ppm on a 8 hour time weighted average.  Nunn showed 450 ppm of nitrous oxide had no effect in rats or operating room personnel.

Rowland et al. looked at the ability of female dental assistants to become pregnant, fucundability.  He first looked at exposure to mercury vapor and found no effect.  He reevaluated the data taking into consideration whether they had been exposed to nitrous.  The assistant were grouped into 5 groups, a control group that were exposed to no nitrous oxide, two groups in offices that used scavenging to remove traces of nitrous oxide depending on low, less than five hours per week or high usage, greater than five hours per week and two additional groups with no scavenging again grouped by high and low usage.  Only the group with no scavenging and high utilization had a statistically significant difference in facundability.   These two studies and a study by Nunn on nurses in operating rooms strongly suggest that if we use scavenging and keep levels below 450 ppm dental office staff is at no risk.

Karakaya concluded that chronic exposure to halothane and nitrous oxide does not affect serum immunoglobulin levels, white blood cell counts or lymphocyte subpopulaltions of anesthetists, anesthetic assistants or anesthetic nurses.

What levels of exposure can be achieved?
Early studies of the Brown masks, a mask within a mask with suction, could achieve levels of 50 ppm. under ideal settings.  OSHA reviewed this data and suggested, since this was achievable it should be our goal.  Donaldson has shown many scavenging devices being used in practicing dental offices can achieve levels in the 40 to 60 ppm range.  NIOSH in a publication of the Centers for Disease Control suggested, in spite of all the research to the contrary, that staff should not be exposed to levels above 25 ppm.  They went so far as to state, "When N2O concentrations are not consistently below 25 ppm, workers should take the following steps to protect themselves: Wear air-supplied respirators."  Staff should wear sealed breathing devices that would be supplied pure outside air when administering nitrous oxide.  They used the retracted study of 50 ppm nitrous oxide with 1 ppm of halothane to justify this stand although this study and the study of 500 ppm have been retracted and are not reproducible.

How can levels be controlled?
It is clear offices should have one of the available scavenging systems on each nitrous oxide oxygen machine.  The system that take gas from the nitrous machine and deliver it to the patient come in 3 basic forms, semi open, open and closed systems.  The systems of the 70s were mostly semi open.  The semi open systems allowed some rebreathing of exhaled gas.  A few systems marketed in the early 80s were open systems.  Open systems allow no rebreathing.  Semi closed systems are rebreathing circle systems that recycle gases.  Circle systems are used routinely in hospital operating rooms but are uncommon in dental offices.

Semi open systems
Scavenging systems take several forms.  Prior to scavenging, all systems could be best described as semi open systems.  The gas left the machine and went to a T that had a reservoir bag and a single 22 mm in diameter hose.  This hose carried gas to a Y that connected to two smaller 11 mm hoses that went around the patientís head and attached to either side of the nose mask.  The nose mask had a pop off valve that vented exhaust gas into the dental operatory usually in the direction of the dentistís face.  Trace gas levels have been reported as high as 5,000 ppm. of nitrous in the dentistís and assistantís breathing zone.  With this system, the patient inhaled pulling gas from the reservoir bag down the tubes to the mask.  When they exhaled, the gas would be blown back down the tubes into the bag.   Once the bag was full the exhaust valve would open and the excess gas would be blown into the room.  There was some rebreathing of exhaled gas that had gone back to the bag.  Exhaled gas is about 3.5% carbon dioxide  (CO2). Rebreathing the exhaled gas lead to higher levels of CO2 in the lungs and blood.  The pH of the blood is determined by the CO2 concentration.  The higher to CO2 the more acidic the blood and the lower the pH.    Lowering the pH of the blood stimulated the inspratory area of the respiratory center of the brain stimulating breathing.  Rebreathing was kept to a minimum by having gas flows from the machine equal to the patient minute volume.  Minute volume can be calculated by multiplying the number of breaths each minute by the volume of each breath.  For the average adult, at rest, the minute volume is about 6 L. per minute.

Open  systems
Some machine manufacturers placed a one way valve down stream from the reservoir bag.  With this valve, the system is now a fully open system and permits no rebreathing.  It was necessary to also add an air valve to the T.  This air valve opened and let air into the system should the machine not be providing a volume of gas equal to the patientís minute volume.   Without the air valve and with low gas flows, if the patient attempted to inhale, the reservoir bag could empty and the patient would not be able to get a breath. At best this is uncomfortable; at worst it is very frightening for an already apprehensive patient. who is using nitrous oxide to help relax.  All exhaled gas was blown out through a pop off valve of the mask into the breathing zone of the staff.

Semi closed systems
Carbon dioxide absorbers and the circle system
A third system that is very common in hospital operating rooms is the semi closed, circle, system.  In this system there is a CO2 absorbing canister placed in a circle.  The patient breathes in pulling gas from the reservoir bag through the CO2 absorber removing any CO2.  The gas goes through a hoses to one side of the mask.  There is a one way valve that prevents the exhaled gas from going back down this tube.  The tube on the other side of the mask carries the exhaled gas back to the reservoir bag where the cycle starts again..  The gas goes around and around this circle.  Each pass through the CO2 absorber removes the metabolic CO2 produced by the patient and exhaled in their exhaled gas, Each exhaled breath has about 300 cc of CO2.  It is theoretically possible, with a completely sealed system, a closed system, not possible in dentistry, to add 300 cc of Oxygen (O2) that was used to produce the CO2 and continue to use the same nitrous oxide over and over again.  In practice there are gas leaks and so it is necessary to add a small volume of nitrous oxide / oxygen mix to replace the lost gas and the metabolized O2.  For the average patient, 1 L. of nitrous oxide and 0.5 L. of O2 are added each minute.

If gas was added without a pop off valve the bag would fill and the patient would be unable to exhale.  For this reason, all systems have pop off valves to allow excess gas to escape.  The valve is usually placed on the CO2 absorbing canister. It was common for those of us who used this system to put suction at this pop off valve to take the exhaust gases out of the operatory.  We had scavenging long before we knew it was necessary.

Scavenging systems
Todayís scavenging systems are predominantly open systems with suction placed at the mask over the pop off valve.  Some systems attempt to suck up additional air that is around the mask and may have traces of nitrous oxide that the patient may have exhaled through their mouth or leaked from around the mask.  One of the first and best systems is a mask, the Brown Mask, within a mask.  The space between the masks is under vacuum.  Another mask has a disk that sits over the pop off valve.  A plastic disk about 3 inches in diameter sucks up the exhaust gas from the pop off valve and brings in surrounding air.   Unfortunately, this mask is rather noisy.  One manufacturers have developed simple relatively inexpensive systems including colored, flavored masks.

A very inexpensive system amounts to an open system. I put together such a system when scavenging first became an issue.  Unfortunately, I did not patent it.  Several manufacturers have marketed this system.  Rather than a Y connector with two hoses leading around the patientís head to the mask, the hose from the bag goes to one side of the mask.  Typically, a 22mm in diameter corrugated hose leads from the machine a step down fitting, leads to an 11mm in diameter tube that goes around the patients head to the mask.  The mask has no pop off valve.  Exhaled gas leaves the mask from the opposite side and goes into a second 11 mm hose, through a step up fitting to a 22 mm. hose that is about a meter long.  There is a one way valve in entry to this hose that only allows gas to flow away from the patient.  The end of this hose is open to the air. Suction is applied to the hose just down stream from the one way valve.  Provided this suction is greater than the patientís minute all the gas is removed.  The typical saliva ejector of a high volume suction system is more than adequate.  In practice, there is always fresh air flowing into the open end of the exhaust hose and being removed along with the exhaled gas.  This system requires two one way valves, two connectors to go from 22 mm to 11mm and one meter of extra hose.  The hole in the mask where the pop off vale goes must be plugged.   I find a plug of impression material works well.  This system can be assembled for under $30.

I still prefer the CO2 absorbers  as they use much less gas, 1.5 L vs. 6 L. per minute.  There is less gas to scavenge and the pop off valve is some distance from dental staffís breathing zone.  All of the systems, now on the market, can be expected to keep levels of trace gas in the 50 ppm range.  Another manufacturer has a device that only turned on the suction when the patient was exhaling.  There were several other more complex systems marketed.  In general, the simpler the system the better it works and the easier it is to setup and maintain. The complex systems have more to go wrong.

All of the available scavenging systems are able to provide a safe environment.  Every nitrous machine must have a scavenging system with adequate suction.  Some systems have small gages to assure adequate suction.  It is very important to make sure the suction system exhausts outside the office.  There should be a reasonable exchange of air in our dental offices. Outside air should be brought into the office by our heating and cooling systems.  It is suggested that the minimum air exchange is 5 changes per hour and it is recommended that 15 to 20 changes is better.  Hoses and connectors should be checked for leaks.  Masks should be selected that fit the patient.  The patient should be discouraged from speaking while receiving nitrous oxide sedation.  If all these suggestions are followed it is possible to stay in the 50 ppm range will below the 450 ppm exposure level suggested by several authors.
 
Unfortunately several jurisdictions including Ontario, Canada  California, Oregon and Washington have set acceptable levels of exposure on the guidelines of ACGIH which are clearly stated to be goals and that they should not be used to set regulations. The ACGIH guidelines were also derived from the flawed retracted studies of Bach. OSHA inspectors in several areas of the country have enforced these lower levels even though OSHA has no regulation beyond a very general statement requiring a safe work place.  One state, Washington, has gone so far as to fine a dentist who exceeded these guidelines.  Unfortunately we are governed and regulated by state statue and federal regulations regardless of how appropriate.  Bureaucrats have been very resistant to review research and set guidelines that reflect the proven accepted scientific studies in this area.  Unfortunately the individual dentist will be prosecuted and fined even though the regulations are wrong.
 

References
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