Oxalic Acid Safety
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Properties of Oxalic Acid

Beekeeper Protection

It cannot be stressed too strongly that oxalic acid is an aggressive substance and needs to be treated with respect. Acid resistant gloves and goggles should be worn and an apron of the type used by mortuary attendants, along with wellington boots that have the tops covered by gaiters so that any falling liquid cannot fall into the boot. A respirator that has specialised organic acid filtering will be required in cases where the acid is sprayed or vapourised. Oxalic acid is also poisonous to humans by ingestion.

Oxalic acid is used in beekeeping as a treatment for varroa mites, this page is intended to help beekeepers to understand the dangers of oxalic acid and the chemical and physical properties that it exhibits.

General Physical Description

Synonyms... Oxalic acid dihydrate, Ethanedioic acid.

US reference numbers and codes


OSHA IMIS Code Number, 1970
CAS No., 144-62-7 (Anhydrous), 6153-56-6 (Dihydrate)
NIOSH, Registry of Toxic Effects (RTECS) Identification Number, RO2450000

Appearance... Transparent, colourless, odourless crystals.
Solubility... 1gm/7ml (water).
Specific Gravity... 1.65
Boiling Point... 149 - 160°C (sublimes).
Melting Point... 101.5°C.
Vapour Density... 4.4 (air=1).
Vapour Pressure... < 0.001 @ 20°C (mm Hg).
Molecular Weight... 126.07.
Chemical Formula... HOOCCOOH.2H2O

Oxalic acid, as purchased is usually in the form of oxalic acid dihydrate, which is a crystalline form with two water molecules attached to each molecule of oxalic acid.

  Oxalic acid molecule

Toxicological Information

Oral rat LD50... 375 mg/kg.
irritation skin rabbit... 500 mg/24hr mild, eye rabbit 250 µg/24hr severe.

Permissible Exposure Limits

OSHA (PEL) for General Industry: 29 CFR 1910.1000 Z-1 Table, 1 mg/m3 TWA
OSHA (PEL) for Construction Industry: 29 CFR 1926.55 Appendix A, 1 mg/m3 TWA
OSHA (PEL) for Maritime: 29 CFR 1915.1000 Table Z-Shipyards, 1 mg/m3 TWA
ACGIH (TLV) Threshold Limit Value, 1 mg/m3 TWA, 2 mg/m3 STEL
NIOSH (REL) Recommended Exposure Limit, 1 mg/m3 TWA, 2 mg/m3 STEL

Health Factors

Corrosive to tissue... When ingested, causing sore throat, burning sensation, abdominal pain, vomiting, diarrhoea, passing stools with blood, drowsiness, loss of feeling in fingers and toes. Oxalic acid removes calcium from the blood causing kidney damage as the calcium is removed from the blood in the form of calcium oxalate. The calcium oxalate then obstructs the kidney tubules.

Harmful if inhaled... Can cause severe irritation and burns to nose, throat, and respiratory tract. Irritations of mucous membranes, eye discomfort and burns, blue colouration of lips, fingernails or skin, shock, collapse, convulsions, kidney damage, pulmonary oedema.

Toxic... May cause burns, nausea, gastroenteritis, vomiting, shock and convulsions. May cause renal damage, as evidenced by blood in urine. Estimated fatal dose for humans is 5 to 15 grams.

Skin Contact... Can cause severe irritation and skin burns. May be absorbed through the skin.

Eye Contact... Oxalic acid is an extreme eye irritant by virtue of being corrosive and physically abrasive.

Chronic Exposure... Causes inflammation of the upper respiratory tract. Prolonged skin contact can cause dermatitis, cyanosis of the fingers and ulceration. May cause kidney damage.

Pre-existing Conditions... Exacerbates pre-existing skin disorders or eye problems, kidney problems or respiratory complaints.

NIOSH (IDLH) Immediately Dangerous To Life or Health Concentration, 500 mg/m3

First Aid Measures

Inhalation... Remove to fresh air. If not breathing, give artificial respiration. If breathing is difficult, give oxygen. Call a doctor immediately.

Ingestion... DO NOT INDUCE VOMITING! Give large quantities of limewater or milk to drink. Never give anything by mouth to an unconscious person. Call a doctor immediately.

Skin Contact... In cases of contact, wipe off excess from skin then immediately flush skin with high volumes of water for at least 15 minutes while removing all contaminated clothing and shoes. Wash clothing thoroughly before reuse unless obviously damaged. Call a doctor or physician immediately.

Eye Contact... Immediately flush eyes with a gentle flow of a copious amount of water for at least 15 minutes, lifting lower and upper eyelids occasionally. Call a doctor immediately.

Additional Notes


American Conference of Governmental Industrial Hygienists (ACGIH).
Chemical Abstracts Service (CAS).
National Institute for Occupational Safety and Health (NIOSH).
NIOSH Pocket Guide to Chemical Hazards, Oxalic Acid.
MSDS Number:- O6044.
World Health Organisation (WHO).
Neurotoxic effects of carambola in rats... the role of oxalate. C. L. Chen, K. J. Chou, J.S. Wang. J. Formos. Medical Association. 101(5), 337-441, 2002.
Sittig's Handbook of Toxic and Hazardous Chemicals and Carcinogens, Fourth Ed., Vol. 2. Norwich, NY, Noyes Publications, William Andrew Publishing, 2002.
Salinas, M.L., Ogura, T. and Soffchi, L., Irritant contact dermatitis caused by needle-like calcium oxalate crystals, raphides, in Agave tequilana among workers in tequila distilleries and agave plantations. Contact Dermatitis 44(2), 94-96, 2001.
Intoxication by star fruit (Averrhoa carambola) in 32 uraemic patients... treatment and outcome Miguel Moyses Neto, José Abräo Cardeal da Costa, Norberto Garcia-Cairasco, Joaquim Coutinho Netto, Beatriz Nakagawa and Marcio Dantas.

Written... 27 October 2005, Revised... 30 October 2005,
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