Fire damage can be divided into two areas, primary and secondary damage.

Primary Damage
The initial damage, the burnt and destroyed effects.

Secondary Damage
The latter effects which often can be prevented. This may include corrosion, perhaps from condensation activating residue chlorides from burnt PVC which create Hydrochloric acid. This can also cause latent and intermittent faults in I.T. or electrical systems, months or years after the event. Corrosion is also possible from evaporation the result of fire fighting materials such as foams or chemical agents, or simply condensation from evaporated water reforming on cooler surfaces. Good mitigation can often prevent this damage, this may mean installing control measures, such as controlling the environment, for example dehumidifiers.

The fuel, available oxygen and time are the key components of fire damage. Fuel can vary from chip pan oil, upholstery plastics and construction materials. Typically, food type fires or protein, such as meat, cause significant odour and are particularly difficult to clean as the film left is often difficult to see. A yellow film may be the only identification, often accompanied by a rancid obnoxious odour. These fire residues are amongst the most difficult to remove due to their obscurity and sealed in grease/odour which requires detailed and professional decontamination. Low oxygen where the fire occurred in a closed environment can result in high pressure air flow carrying soot and penetrating permeable materials and above all areas of a lower pressure such as cupboards, closets, and voids. The longer the smoldering takes place, the higher the penetration. The low oxygen prevents complete combustion and therefore the soot particulate is often wet or greasy and difficult to remove. High oxygen fires usually result in little soot which is relatively easy to remove. Soot follow recognised routes, generally from high to low pressure and hot to cold, significantly from the source to distant areas such as cupboards and draws, often falling out of the air as they reach lower or different pressure areas, this can be typically seen under doors, between rooms, corners etc. This may be seen as sooty deposits where the soot falls out of the air due to different carrying capacities of the air.

Soot particles are often ionised or polarised and can create smoke webs to form smoke chain particulates, seen as corner ‘cobwebs’. As the building heats up, porous materials expand and their pores open, soot enters, as the surfaces cool, the pores close, effectively locking in the odour. Following inappropriate cleaning and redecoration the pores again open as the building heats up after re-occupancy, resulting in the pores expanding which releases odour. The chemical composition of soot varies in every fire situation. One by-product of burning PVC is chloride gas which combines with moisture in the air to form hydrochloric acid. This gas phase may affect electronic, electronic and metallic surfaces, and sometimes cause fading or yellowing of plastics. Ventilation of the fire scene or debris removal is an effective first step to clean up. Thermal fogging and often deodorizing techniques are temporary fixes and should not be relied upon as a remedy. Decorating should never be a first choice of restoration as bleed through is very likely at some point in the future. Thorough cleaning and neutralising of both the deposits and odour are required prior to any redecoration. Chloride testing is recommended to identify excessive acidic deposits which should be neutralised, especially where electronics or micro-volt systems are involved.

Following adequate risk and hazard assessments the following steps should be followed:

• Ventilate the area to remove soot and odour.
• Remove burnt debris to reduce odour source.
• Install dehumidifiers to control moisture in the air (relative humidity), especially where water was used to extinguish the fire.
• Wipe all metallic finishes with cooking oil to prevent rust, staining.
• Clean plastic or surfaces such as PVC windows and white painted surfaces using a mild alkali detergent to remove possible acidic soot which may activate with moisture in the air (humidity) and cause permanent staining.
• Undertake triage assessments to clean or remove all contents as quickly as possible and in order of value.
• Deodorants shouldn‘t be used as they mask odour which is a significant indicator of health concerns.
• If it smells bad it will probably result in a sore throat or cough. Either wear respiratory protection or leave site.
• Where staining to surfaces cannot be removed you may consider the use of specialist paint to obliterate the stain and anti bleed characteristics are essential. Typically these paints are lacquers or oil based.

The different types of fire will require different cleaning techniques, typically high oxygen fires will result in dry dusty type soot whereas slow burn low oxygen will result in greasy wet deposits which easily smear. As previously mentioned kitchen type fires often result in the most difficult of all residues and odour to remove. The cleaning regime must take into account these variations and the following methods are guidelines:

1. High oxygen. Use dry sponges to remove initial deposits and follow with a low alkali detergent and rinse.
2. Low oxygen. Do not dry sponge as these may cause smears and spread. Recommend high alkali detergents, warm water and wash down, and remember to rinse thoroughly as residue may affect following paint applications.
3. Protein or Kitchen fires. Thorough detailed cleaning is required in small sections, which can be identified. This is required as visible residue is unlikely and therefore requires competent controls and inspection. Remember that usually kitchen cabinets and all contents may need to be removed to allow access to hidden areas.
4. Heavy residue. This may require mechanical cleaning which can be undertaken in various ways, which include;
5. Low pressure sand blasting
6. Soda Bicarbonate blasting
7. Dry Ice blasting
8. Power washing with steam following chemical application and adapting dwell periods prior to removal.
9. Non destructive rubber blasting

Invariably the heat generated from even small fires will cause laminar air flow which will carry smoke and odour, (soot) to areas far away from the fire seat. Smoke usually goes from hot to cold areas, high pressure areas to low pressure areas, (drawers, corners, cupboards, wardrobes). This can often be seen under doors. Look for soot, (odour) and secondary damage in areas far away from the initial fire seat. To prevent secondary damage and spread consider installing environmental controls such as negative pressure and temperature, humidity controls.