Working Safely

Formalin can be used quite safely if some simple precautions are followed. Unfortunately, every once in a while an accident occurs, and it is quickly appreciated how important these precautions can be.

The major danger from formalin is asphyxiation. Formaldehyde gas is given off by open formalin and stimulates a choking reflex and an inability to breathe. Spills of surprisingly small amounts of 100% formalin can produce enough formaldehyde in the atmosphere to cause this choking reflex, invariably followed by a deep breath during which more formaldehyde is breathed in causing even greater choking. This choking-breathing cycle is repeated over and over again and can eventually lead to dizziness and collapse in the worst cases. The higher the concentration of formaldehyde the more quickly and more intense the distress, and the greater the likelihood of collapse.

This is a very important rule:

If you have any difficulty breathing because of formaldehyde
Leave the area immediately

There are also other effects, caused by the formaldehyde in the atmosphere dissolving into the moisture around the eyes, and into the saliva and nasal secretions. However, these effects are an irritation, but not serious dangers unless continued for long periods.

General handling
Since it is a fixative, it should be self evident that skin contact should be avoided. Gloves should always be worn, and bare hands or fingers should never be plunged into containers of formalin to retrieve tissue pieces. Doing so can lead to dermatitis. In fact, some people have a sensitivity to formalin and quickly develop a rash on exposure. Those affected in this way should take extra precautions against exposure, as once a sensitivity develops it never goes away. It is preferable for such people to change to another laboratory discipline if possible.

Wear a properly fitted mask with filters specifically designed to remove formaldehyde if working in an atmosphere with detectable amounts of it, such as when removing fixative from tissue prior to disposal or when making up quantities of fixative in bulk. General purpose carbon filters are not appropriate for this, and those designed specifically for formaldehyde are required. In some circumstances, such as during gross sectioning and dictating when exposure can be expected to be limited, an alternative to a mask is a laminar flow hood. It is important to ensure that the air flow is strong enough to remove all formaldehyde fumes. If formaldehyde can be detected then the air flow is not adequate. We appear to be quite sensitive to formaldehyde, and our noses and throats are likely the best detection system in practice.

Neutralising formic acid
On standing, formalin solutions deteriorate to produce formic acid. Since this can produce acid formaldehyde hematin (formalin pigment) in sections, it is a common practice to neutralise the formic acid. The modern way is to buffer the diluted solution with phosphates (Sorenson's buffer), but in the past it was often done by storing the formalin over marble chips. Any formic acid present reacts with the carbonate, producing carbon dioxide and calcium formate, and the formalin remains unacidified. Although this is a simple, cheap and effective procedure it has the potential for a serious problem – the possibility of an explosion when marble chips are put into containers of 100% formalin. As carbon dioxide is generated the pressure can increase enough to cause the container to explode, showering concentrated formalin (and possibly glass) all over the area. For personal safety never neutralise concentrated formalin. Add marble to diluted formalin only, and always use a lid that can permit any gas generated to escape.

Formaldehyde is a suspected carcinogen, so exposure should be minimised at all times. The cumulative effects of long term repeat exposure to high levels of either the fumes or fluids are not well documented. There are anecdotal reports of some pathologists having eye, nose and skin problems following long term exposure to formalin during gross sectioning. The fumes have either dissolved in their tears and the secretions of mucous membranes, fixing the tissues involved in-situ, or repeated skin contact has caused chronic skin damage. It is far better to err on the side of caution and work with formalin in areas which have an adequate air exchange systems. By “adequate” is meant an air extraction system with enough capacity to remove all traces of the gas so that it is undetectable. Extracting the fumes, of course, does not stop physical contact. Gloves should be worn and sleeves from coats should not drag in fixative (wear short sleeves or roll them up a little).

10% formalin spills
Small volume spills of 10% formalin are usually not too problematic, and usually do not cause distress in breathing. Cleanup while wearing a properly fitted mask with the correct filters attached is usually all that is required. Note the number of times “usually” was used. If there is any difficulty at all be very cautious. There is nothing to be gained by continuing to work in an objectionable atmosphere.

Note that spills involving large volumes of 10% formalin should be treated as being a significant hazard, since they can be as dangerous as spills of 100% formalin. This is particularly so if in a confined space. If the fumes stimulate choking and a mask is not available, or if the mask does not completely remove the formaldehyde fumes then this should be treated the same way as a 100% formalin spill, given immediately below.

100% formalin spills
A small spill can be defined as one that does not cause distress in breathing, or one in which the fumes are contained by a properly fitted mask with the correct filters attached. Usually these are also small in volume as well, being just a few mL. They also pose no significant problem, and may be cleaned up as a 10% formalin spill.

Larger spills pose a significant problem. As little as one litre of 100% formalin can saturate the atmosphere with formaldehyde in a room and make it impossible to breathe properly. In these conditions the usually available type of mask with canisters designed to remove formaldehyde are useless. A self contained breathing apparatus (the ones with air cylinders) is required. Most laboratories do not have these, so someone has to hold their breath while working (absolutely to be avoided), or a local hazardous materials team (hazmat) has to be asked to deal with it. In most cases hazardous materials are dealt with by local Fire Departments, since using the necessary breathing apparatus is a normal requirement of the job. Call in the Fire Brigade!

It is probably a good idea to limit the decision to call in the Fire Department to the Supervisor of Anatomic Pathology or the Chief Technologist of the laboratory and others of that level of responsibility, thus diminishing the possibility of unnecessary calls. On the other hand, safety comes first. Better to be embarrassed than dead.

The actual physical cleanup of spilled formalin depends on the type of spill. For small spills, wiping with a paper towel or flushing with water is adequate, depending on how much was spilled. Neutraliser impregnated absorbant towels and pads are available commercially for this purpose. If anything more than a small volume is involved it may cause some problems as the formalin can flow into every nook and cranny when spilled, and missing any just lets the noxious fumes percolate away for some time. More, rather than less, water is always recommended.

Using water is not recommended for other than minimal spills. For larger spills there are two very effective cleanup systems. One uses urea, the other ammonia. Either is effective, although the use of ammonia or salts of ammonia is usually faster and less problematic once the formalin smell dissipates.

Urea forms a compound with formaldehyde (urea-formaldehyde resin) which is a sticky solid when fresh, but hardens into a plastic-like hard material after some time. This has to be scraped up and may take some effort to do so. The material is used industrially as an adhesive, which may give some indication of the firmness with which it attaches to benches and floors. Formalin neutraliser granules are available commercially as spill kits, and this is a very convenient way to obtain them.


Ammonia reacts with formaldehyde to produce a non offensive compound, according to the following:

H2C=O + NH3  =  H2C=N-H + H2O.

The ammonial system may be applied in a few different ways, depending on the nature of the cleanup required.


Further information
From the Wikipedia article about formaldehyde.

Toxic Link
ILO safety data
ILO safety data

Formaldehyde hazards
Formaldehyde gas
Liquid formalin



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