Formalin: safe storage and disposal
Formalin is a water-based solution of formaldehyde (CAS 50-00-0), usually with methanol added to slow polymerisation. It is the working fixative of histology, in the stock form and in neutral buffered formalin. It also sits in many old school biology collections, preserving specimens that nobody has looked at for thirty years.
Its GHS signal word is Danger. The hazard statements on our record are H302 Harmful if swallowed, H311 Toxic in contact with skin, H314 Causes severe skin burns and eye damage, H317 May cause an allergic skin reaction, H330 Fatal if inhaled, H341 Suspected of causing genetic defects and H350 May cause cancer.
- CAS number
- 50-00-0
- Form
- Solution
- Storage class
- Toxic
- Signal word
- Danger
- Usual EWC code
- 16 05 06*
Corrosive
Acute toxicity
Serious health hazard
What to do this week
- Check the label is readable and write down the opened date if it is not already there.
- Check it is stored as toxic stock and apart from the items listed under "Do not store with".
- If you no longer need it, add it to your disposal list so it goes on the next collection.
Safe storage of formalin in a school or lab
Formalin belongs with toxic stock, because the toxic and carcinogenic hazards govern. Keep it in a closed, labelled container, in a locked store or a dedicated cabinet with good ventilation, and below eye level so a spill does not reach the face. Keep it away from acids and oxidisers.
Your COSHH assessment sets the working controls, including local exhaust ventilation for grossing and specimen handling, and exposure monitoring where your assessment calls for it. Sensitisation is a recognised concern among histology staff, so skin contact matters as well as vapour.
Very cold storage can cause solids to drop out, and heat raises the vapour. A steady, moderate temperature suits it best. Specimen pots deserve attention as well, as they are often stored in racks and fridges outside the main chemical store, and they can easily get forgotten.
How to tell formalin has deteriorated
Old formalin often shows a white solid at the bottom or around the neck. This is paraformaldehyde, formed as the formaldehyde polymerises over time, especially when cold. Solution strength will have changed, which makes it unreliable as a fixative. Old stock can also become more acidic.
For the container, look for perished seals, corroded metal lids and cracked jars. Old specimen jars are a known problem, because the seal fails slowly and the fluid evaporates, leaving a dried specimen and a smell. A bottle showing white solids should not be warmed, stirred or filtered to recover it. List it for disposal, and if a jar is leaking or badly degraded, keep the room closed and get specialist help.
How formalin must be disposed of
Formalin goes by specialist collection. We do not publish a sewer route. HTM 07-01 says liquid chemical wastes should never go down the drain.
| Setting | Usual List of Wastes code | Hazardous |
|---|---|---|
| Laboratory chemical, including school and university | 16 05 06* | Yes |
| Discarded organic chemical, not in a laboratory pack | 16 05 08* | Yes |
| Human healthcare setting | 18 01 06* | Yes |
| Veterinary setting | 18 02 05* | Yes |
| Chemically preserved anatomical waste, human healthcare | 18 01 06* with the anatomical code, 18 01 02 or 18 01 03* | Yes |
The last row catches people out. The healthcare classification guidance gives chemically preserved anatomical waste a pair of codes, with the second depending on whether the tissue is infectious. Agree the wording with your contractor before a run of pots goes out.
For transport, formaldehyde solutions usually travel under UN 2209, packing group III, though flammable solutions carry UN 1198. The precise entry depends on the product, so check its data sheet. Departmental streams are handled through clinical and laboratory waste collections.
| EWC code, laboratory chemicals | 16 05 06* |
|---|---|
| Hazardous waste | Yes |
The code depends on where the waste came from and what it is mixed with. Use the waste code finder to see why.
Do not store formalin with
- Hydrochloric acid can react with formaldehyde to form a potent carcinogen. Keep them apart in storage and in waste, and make sure this is understood wherever decalcifying fluids are used near fixative.
- Sodium hypochlorite should not be used near it. A bleach bucket next to a formalin sink is a common error.
- Phenol reacts with formaldehyde. Hydrogen peroxide, nitric acid and potassium permanganate are also incompatible.
- Perchloric acid is hazardous with organic materials.
- Water-reactive materials such as sodium, potassium, lithium and calcium metals, as well as hydrides and calcium carbide, must stay away from any water-based stock.
Store it apart from oxidisers and acids as classes.
- OxidiserToxic and oxidising stock are stored apart. A reaction could release toxic vapours.
- AcidToxic stock is stored apart from acids. Several toxic salts release toxic gas in contact with acid.
Named chemicals to keep apart
- Acetic acidToxic stock is stored apart from acids. Several toxic salts release toxic gas in contact with acid.
- Calcium carbideReacts with water, releasing heat and often a flammable or corrosive gas, so keep it away from all water based stock.
- Calcium hypochloriteToxic and oxidising stock are stored apart. A reaction could release toxic vapours.
- Calcium metalReacts with water, releasing heat and often a flammable or corrosive gas, so keep it away from all water based stock.
- Hydrochloric acidFormaldehyde reacts with these materials. With hydrochloric acid it can form a potent carcinogen.
- Hydrogen peroxideFormaldehyde reacts with these materials. With hydrochloric acid it can form a potent carcinogen.
- Lithium aluminium hydrideReacts with water, releasing heat and often a flammable or corrosive gas, so keep it away from all water based stock.
- Lithium metalReacts with water, releasing heat and often a flammable or corrosive gas, so keep it away from all water based stock.
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- Nitric acidNitric acid reacts violently with organic solvents and can ignite them or build pressure in waste bottles.
- Perchloric acidPerchloric acid forms explosive mixtures with organic material.
- PhenolFormaldehyde reacts with these materials. With hydrochloric acid it can form a potent carcinogen.
- Potassium metalReacts with water, releasing heat and often a flammable or corrosive gas, so keep it away from all water based stock.
- Potassium nitrateToxic and oxidising stock are stored apart. A reaction could release toxic vapours.
- Potassium permanganateFormaldehyde reacts with these materials. With hydrochloric acid it can form a potent carcinogen.
- Sodium hydrideReacts with water, releasing heat and often a flammable or corrosive gas, so keep it away from all water based stock.
- Sodium hypochloriteFormaldehyde reacts with these materials. With hydrochloric acid it can form a potent carcinogen.
- Sodium metalReacts with water, releasing heat and often a flammable or corrosive gas, so keep it away from all water based stock.
- Sulfuric acidToxic stock is stored apart from acids. Several toxic salts release toxic gas in contact with acid.
Common quantities of formalin collected
Histology labs may run a steady flow of containers from cut-up rooms, along with spent fixative from processors. Theatres and clinics send out small pre-filled pots. Schools tend to hold older bottles and jars of specimens, often in very small numbers but with a high degree of uncertainty about their condition.
Usually held as: 37 to 40% formaldehyde.
Part-used bottles and full containers are both fine. Tell us the size and how many, and an estimate is enough for a first price.
Formalin in different sectors
Pathology and histology. Keep fixative, spent fixative and pots with tissue as separate lines. Our page on pathology and histology laboratories goes further into how those streams fit together.
Theatres and clinics. Pre-filled pots returned unused are chemical waste, not clinical waste. HTM 07-01 keeps waste chemicals out of clinical and offensive streams.
Schools and colleges. Preserved biology specimens are the classic find. Check jars as part of your stock check, and include their age on your list. Do not top them up or transfer them.
Veterinary practices. Specimen pots follow the animal healthcare codes, including 18 02 05*.
Hazard data from PubChem GHS classification, retrieved 31 October 2026. Guidance only. Check your own risk assessment and your supplier's safety data sheet.
Hazard statements
- H302 Harmful if swallowed.
- H311 Toxic in contact with skin.
- H314 Causes severe skin burns and eye damage.
- H317 May cause an allergic skin reaction.
- H330 Fatal if inhaled.
- H341 Suspected of causing genetic defects.
- H350 May cause cancer.
Precautionary statements
- P203 Obtain, read and follow all safety instructions before use.
- P260 Do not breathe dust, fume, gas, mist, vapours or spray.
- P261 Avoid breathing dust, fume, gas, mist, vapours or spray.
- P264 Wash hands and exposed skin thoroughly after handling.
- P270 Do not eat, drink or smoke when using this product.
- P271 Use only outdoors or in a well-ventilated area.
- P272 Contaminated work clothing should not be allowed out of the workplace.
- P280 Wear protective gloves, protective clothing, eye protection and face protection.
- P284 In case of inadequate ventilation wear respiratory protection.
- P301+P317 IF SWALLOWED: Get medical help.
- P301+P330+P331 IF SWALLOWED: Rinse mouth. Do NOT induce vomiting.
- P302+P352 IF ON SKIN: Wash with plenty of water.
- P302+P361+P354 IF ON SKIN: Take off immediately all contaminated clothing. Immediately rinse with water for several minutes.
- P304+P340 IF INHALED: Remove person to fresh air and keep comfortable for breathing.
- P305+P354+P338 IF IN EYES: Immediately rinse with water for several minutes. Remove contact lenses, if present and easy to do. Continue rinsing.
- P316 Get emergency medical help immediately.
- P318 If exposed or concerned, get medical advice.
- P320 Specific treatment is urgent (see supplemental first aid instruction on this label).
- P321 Specific treatment (see supplemental first aid instruction on this label).
- P330 Rinse mouth.
- P333+P317 If skin irritation or rash occurs: Get medical help.
- P362+P364 Take off contaminated clothing. And wash it before reuse.
- P363 Wash contaminated clothing before reuse.
- P403+P233 Store in a well-ventilated place. Keep container tightly closed.
- P405 Store locked up.
- P501 Dispose of contents and container in accordance with local, regional, national and international regulations.
- P262 Do not get in eyes, on skin, or on clothing.
- P264+P265 Wash hands and exposed skin thoroughly after handling. Do not touch eyes.
- P301+P316 IF SWALLOWED: Get emergency medical help immediately.
- P317 Get medical help.
- P361+P364 Take off immediately all contaminated clothing. And wash it before reuse.
Substance as classified, dilutions may differ.
Questions
Our stock formalin has a white deposit at the bottom. Has it gone off?
The deposit is paraformaldehyde, brought on by cold storage or age. Its stated strength is no longer reliable, so it should not be used to make up fixative for diagnostic work. Do not warm or stir it. List it as formalin with a deposit when arranging collection.
How do we dispose of used fixative that contains tissue?
Describe it as used fixative containing tissue, and do not list it as plain formalin. Keep it in separate closed containers from unused stock and record the volume. Tissue changes how the waste is described, so tell the collection team exactly what it holds and where it came from.
What should a school do with old formalin specimen jars?
Count them and note what each appears to hold, and which ones have failed seals or low levels. Leave them closed and do not try to transfer specimens or top up the jars. List them separately from any stock bottle, as specimens are a different item for packing.
Is our weak fixative as hazardous as concentrated formalin?
A dilution may be less hazardous than the concentrated solution, but it should not be assumed safe. It still gives off formaldehyde vapour, and it is still a sensitiser concern for staff. Its label should state the strength, so that it can be described accurately when collected.
