The Elusive Odor Theshold for Hydrogen Peroxide
Hydrogen peroxide vapor has no smell and is imperceptible at concentrations below a few hundred ppm. Therefore odor is an unreliable indicator of the presence of hydrogen peroxide and in applications where hydrogen peroxide vapor is present a continuous monitor should be used for occupational safety.
When thinking about whether continuous monitors are needed for hydrogen peroxide sterilizers, hydrogen peroxide aseptic lines and other uses of hydrogen peroxide, the question arises as to what is the concentration that people will be able to perceive the hydrogen peroxide vapor and whether they can perceive the hydrogen peroxide vapor below the concentration it becomes hazardous.
The concentrations at which hydrogen peroxide vapor becomes hazardous are well known. NIOSH has issued an immediately dangerous to life and health (IDLH) limit of 75 ppm and OSHA has set the permissible exposure level for occupational exposure at 1 ppm calculated as an 8 hour time weighted average (TWA). Washington State and Hawaii have set short term exposure limits (STEL) of 3 ppm, calculated as a 15 min TWA and the UK has a STEL of 2 ppm. The PEL was based on chronic exposure tests in animals which found long term effects from exposure to even a few ppm.
So far I have been unable to find a reference that gives an odor threshold for hydrogen peroxide. Some references say that hydrogen peroxide has no odor, others that it has a slightly sharp odor, or sharp odor and bitter taste. Jon Ruth in his review  of odor thresholds did not give an odor threshold, but said the odor was slightly sharp and became irritating at 150 mg/m3 (108 ppm).
Solvay, one of the larger manufacturers of hydrogen peroxide says “hydrogen peroxide has very little odor at the lower concentrations found in consumer products” and most people seem to agree that at low concentration there is no odor, at 3%, 6% and even up to 30%, though others report a slightly acid odor at this concentration. Personally, I smelt nothing from bottles of either 3% or 30% solution.
Solvay went on to say that “hydrogen peroxide has a slightly sharp, pungent odor in higher strength industrial concentrations” and others report that pure hydrogen peroxide has a smell similar to nitric acid, though FMC (now peroxychem) say that 90% H2O2 is odorless, as did Thenard, the discoverer of hydrogen peroxide in 1832. Therefore, in summary hydrogen peroxide has little or no odor at concentrations below about 30%, but at very high concentrations some people perceive a sharp smell, but others do not.
Schumb et al wrote that “It is difficult to characterize the smell of hydrogen peroxide. In fact, it is probably to be questioned whether it affects the olfactory cells or merely stimulates the general nerves of the nasal membrane. Concentrated hydrogen peroxide manifests little odor unless occasion is deliberately taken to inhale near the surface of the liquid in a container or unless it has been spilled extensively. The sensation perceived is then reminiscent of ozone or of the halogens. … If circumstances arise to cause the dispersion of considerable hydrogen peroxide in the air, as a mist, there ensues after a short time considerable irritation. Prolonged breathing in such atmosphere induces gasping such as is encountered with ammonia or sulfur dioxide. This is accompanied by a sharp, burning sensation in the nasal passages on inhalation and exhalation.” The much greater acute effect of hydrogen peroxide mists compared to gas probably corresponds to the much higher amount of hydrogen peroxide delivered.
The equilibrium concentration of hydrogen peroxide vapor over 30 % H2O2 solution is about 470 ppm, based on its vapor pressure at 25 oC. which is well above the OSHA PEL and the various STELs discussed above. Personally, I found there was no smell from 20 ppm hydrogen peroxide vapor, and I have no desire to test higher concentrations.
The ATSDR (part of the U.S. Department of Health and Human Services) stated “Because it [hydrogen peroxide] is nearly odorless and nonirritating except at high concentrations, persons may not be aware of its presence. No odor threshold was located for hydrogen peroxide (the OSHA PEL is 1 ppm). Detection of odor does not provide adequate warning of hazardous concentrations.”
Therefore if one is using hydrogen peroxide where there is a risk of exposure to the vapor, odor is not a reliable method to detect if the hydrogen peroxide concentration becomes too high and so a continuous monitor for hydrogen peroxide is needed.
In some applications such as when using sterilizers there is an odor that people sometimes associate with hydrogen peroxide. As far as I know no studies have been performed to identify the origin of this odor, and it is probably the result of partially oxidized volatile organic compounds, perhaps derived from the oxidation of minor components of sterile wraps or other polymeric materials. Similarly, odors can persist after ozone treatment of rooms for much longer than ozone will be present, probably also due to the formation of partially oxidized VOCs. Whatever the source of the smell, people who work near hydrogen peroxide sterilizers who have a hydrogen peroxide monitor present can be glad that it is not hydrogen peroxide vapor. Those who do not have continuous monitor for hydrogen peroxide may also be glad the smell is not hydrogen peroxide but they will still wonder how much hydrogen peroxide vapor they are breathing in.
 “Inhalation Toxicity of Ninety percent hydrogen Peroxide Vapor” F.W. Oberst, C.C. Comstock, E.B. Hackley, Archives of Industrial Hygiene A. (1954), p 319.
 “Odor thresholds and Irritation Levels of Several Chemical Substances – A Review” J.H. Ruth, Am. Ind. Hyg. Assoc. J. , March 1986, A-142.
 A Comprehensive Treatise on Inorganic and Theoretic Chemistry, J.W. Mellor, Vol. 1, p929 (1927), citing J. Traube. Ber. 40, 138, (1907)
 Hydrogen peroxide by W.C. Schumb, C.N. Satterfield, R.L. Wentworth, Publ. ACS Monograph series (1955)
 Lange’s Handbook of Chemistry, 12th Ed.p10-28