Photograph of a Drager hand pump used to measure carbon dioxide levels in the environment. Toxicity of Oxygen Gas Exposure, O2
Poisoning Symptoms, Oxygen Gas Exposure Limits, and Links to Toxic Gas Testing Procedures

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Effects of exposure to abnormal levels of oxygen gas:

This document discusses the toxicity and exposure limits for exposure to oxygen gas O2. We also describe the normal levels of oxygen in the atmosphere and include a chart of the history of oxygen levels on earth.

We give references and explanation regarding toxicity of oxygen, based on literature search and obtained from the U.S. government and expert sources. This text may assist readers in understanding these topics. However it should by no means be considered complete nor authoritative. Seek prompt advice from your doctor or health/safety experts if you have any reason to be concerned about exposure to toxic gases.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

O2 POISONING SYMPTOMS - Oxygen Gas poisoning symptoms

Photograph of a Drager hand pump used to measure carbon dioxide levels in the environment.

Recommendations for tools and methods for detecting gases in buildings or outdoors

The photo shows a Drager colorimetric gas detection tube (also called a "color detector tube") used to test levels of a very wide range of specific gases in air. In an indoor air test (in our laboratory) this particular detector was not being used to measure oxygen, but rather carbon dioxide.

As the blue-stained portion of the tube shows, we found that the CO2 level was about 600ppm which is typical of indoor air and is an acceptable and safe level.

Colorimetric gas detection tubes such as those sold by Drager (or Draeger), Gastec, (two that we use predominantly) and by Kitagawa, and pumps from Drager, Gastec, Komyo Rikagaku Kitagawa, and RAE all work on a similar principle: a measured volume of gas (or air) is drawn through a tube which contains chemicals which change in color in response to the presence of a specific target gas (or range of gases) present in the sample.

By knowing the volume of gas or air sampled, the amount of color change read on a linear scale on the colorimetric gas detection tube can be translated into a very accurate measurement of level of gas present, described in percentage of the total air or in parts per million (PPM).

Other specialized gas detection methods include use of solid state circuity, CMS chips, and special instruments which may be designed to give a quick alarm or a reading in PPM for specific gases. Other gas and air monitoring equipment use pumps which collect and insert a specific volume of air into a vacuum container for later analysis.

We've found that for typical field use, the colorimetric gas detector tube method is extremely convenient and very accurate, and it presents minimal requirements for instrument calibration.

What happens if a human breathes too much oxygen rather than too little?

Since we've discussed displacement of and reduction of the level of oxygen by an increase in CO2 or by dangerous carbon monoxide CO levels indoors in occupied buildings, (see links for these topics at the "More Reading" links at the bottom of this article ) let's take a look at the toxicity of oxygen itself and at what happens if the mix of gases in air changes in the opposite direction, increasing rather than decreasing the level of oxygen above normal amounts.

Where does this occur? Two examples of exposure of humans to unusually high levels of oxygen occur in "oxygen bars" and in hyperbaric medicine.

Technical note on oxygen toxicity: What about too much oxygen in air:

While high levels, even 100% oxygen are used for medical purposes in some cases (hyperbaric oxygen patients breathe 100% O2 at pressures above 2 to 2.4 ATM for 40-60 minutes),breathing high levels of oxygen can be toxic for humans and other animals. (Toxicity of excessive oxygen will depend on the oxygen level, the duration of exposure (breathing it), and individual characteristics).

But breathing high O2 air too long can produce toxic free radicals, producing effects that can be observed clinically in the human lungs or central nervous system. The current recommended limits of oxygen exposure are up to 24 hours with 100% O2, and up to 48 hours with 60% O2. OSHA does not have a permissible exposure limit (PEL) for oxygen.

What are the symptoms of exposure to elevated levels of oxygen?

The symptoms of excessive oxygen exposure include the following: (some or all of these might occur)

What are the medical effects or symptoms of prolonged exposure to 100% oxygen?

If a person (say a medical patient undergoing hyperbaric treatment at high levels of oxygen and at increased pressure) is breathes 100% oxygen for more than a day (24 hours) s/he may show the symptoms we listed just above, and also the following:

Reader Question: At what level does Oxygen become toxic ?

05/04/2015 Anonymous said:
What is the ppm at which oxygen becomes toxic?


Anon, OSHA has not set a PEL for Oxygen.

Bert originally described that CNS toxicity occured at oxygen pressures of > 3 ATA, it may however, occur at lower pressures if exposure is prolonged. Note that this is at three atmospheres of pressure. Though early manifestations are variable, twitching of perioral and small muscles of the hand is a fairly constant feature. Intense peripheral vasoconstriction due to hyperoxia and diaphragmatic twitching can result in facial pallor and ‘cogwheel’ breathing, respectively4,7.

Continuation of exposure can lead to vertigo and nausea followed by altered behaviour, clumsiness, and finally convulsions. The convulsions are usually tonic-clonic, and the patient has no memory of the crisis4,8. A neurogenic pulmonary oedema concomitant with the convulsions has also been reported .

The factors aggravating the CNS toxicity are raised pCO2, stress, fatigue, cold, and dietary deficiency of trace elements such as selenium, zinc and magnesium1,4,10. CNS toxicity is mainly due to oxidation and polymerisation of -SH groups of enzymes leading to their inactivation, which in turn results in cellular damage.

... Pulmonary effects of oxygen toxicity can occur after a prolonged exposure to oxygen > 0.5 ATA. Symptoms appear after a latent period whose duration decreases with increase in pO2 . In normal humans, the first signs of toxicity appear after 10 hours of oxygen at 1 ATA. (Chawla (2001) - According to Patel et als (JIACM 2003; 4(3): 234-7)

What is the Normal Level of Oxygen in Air?

The normal level of oxygen in outdoor air is just under 21%. This graph from Wikipedia (ret. 2018 07 07) illustrates the history of variation level of oxygen in air on the earth. Before there were people the level was as low as 5%, and its peak was 35%.

Graph of the history of oxygen levels on earth - WolfmanSF cited in this article at

[Click to enlarge any image]

Original source of the Wikipedia graph above: LordToran derivative work: WolfmanSF (talk) - Sauerstoffgehalt-1000mj.svg translated from the original German.

Dieses Diagramm stellt die Sauerstoffkonzentration in der Erdatmosphäre im Lauf der letzten Jahrmilliarde dar. Als Referenz zeigt die gestrichelte rote Linie die heutige Konzentration von 21% an. ...

This graph represents the oxygen concentration in the Earth's atmosphere over the last few billion years. For reference, the dashed red line indicates today's concentration of 21%.

It should be noted that the periods in which past fluctuations in the O2 concentration occurred can often only be estimated relatively roughly. Therefore, the diagram does not claim to be exact, but can give only a rough overview of the evolution at the level of geological periods.

This diagram is based on averages or matches from several publications - cited in the original article - Ed.

Explanation of the curve:

After the outgassing of oxygen from the oceans had already begun in the Precambrian, the oxygen content jumped abruptly from about 3% to 12% at the beginning of the Cambrian, as finally all oxygen sinks were saturated. At about the same time, the Cambrian explosion started.

During the Silurian and Devonian plants conquered the land, while the animal kingdom was still confined almost exclusively to the water.

This triggered another rapid and continuous increase in oxygen concentration. In the following Carboniferous it had reached 35%, which favored gigantism, especially in insects. Amphibians and the first reptiles have also settled in the country.

Massive volcanic activity caused not only a drop in oxygen content to 15% at the Permian-Triassic transition, but also the largest mass extinction of the Earth's history.

The oxygen concentration recovered over a long period and reached 26% in the middle of the Jurassic, probably even 30% in the Cretaceous. During this time lived the largest dinosaurs.

The end of the Cretaceous is marked by an impact event and a climate change with mass extinction. 40 million years ago, the oxygen content was only 23% and had reached its current level of 21% 25 million years ago. Since then, it has been constant, except for fluctuations in the per thousand range.

The nice pair of pie charts below, also from Wikipedia portrays the mix of gases in air. Op. Cit. ret- 7/7/2018

[Click to enlarge any image]

Percentages of gases that make up air - at Wiki cited in this article


Continue reading at GAS EXPOSURE LIMITS & STANDARDS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see CONVERT PPM to % CONCENTRATION To convert between % and ppm

Suggested citation for this web page

OXYGEN - O2 at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Questions & answers or comments about Oxygen gas exposure limits and toxicity for humans.

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman