Blast cleaning is a group of cleaning methods in which abrasive media particles are introduced into an air (or sometimes water) stream, which is then directed at a surface to clean it. There are many media types in use, including soda (sodium bicarbonate), as you mentioned. Media types include steel grit, water ice chips or flakes, plastic chips, walnut shells, dry ice, sponges embedded with a variety of media, and others.
In this article, we will first discuss the characteristics of blast cleaning in general, and then those of two specific media.
Air pressure, volume, and the distance the media has to travel to the surface after injection all significantly affect the aggressiveness of blast cleaning methods, as do the density, hardness, weight, size and other characteristics of the media used. The ability to adjust these many variables individually makes blasting an extremely versatile system of cleaning. Some blast methods are so gentle they can remove graffiti from painted surfaces without damaging the paint beneath. Others are so aggressive that they can deeply "profile" a steel plate surface for better paint adhesion.
Two main uses for blasting have gotten a lot of attention recently in the cleaning/restoration industry. These are: removing charred wood and smoke residue from framing after a fire, and removing mold growth from wood framing (and sometimes other surfaces) during mold remediation. In both applications, blasting is very effective at thoroughly cleaning the intricate surfaces which are so labor-intensive to sand, scrape or wire brush manually.
When media blasting is used for these purposes, we should keep in mind what blasting actually does. Most blast media will do a great job of removing char, smoke or mold growth from wood framing. However, when the contaminant comes off the surface, it does not disappear. Instead, it aerosolizes and creates worker exposure levels that probably exceed those created by any other method of cleaning.
High-pressure water washing is similar in many ways to blast cleaning methods, using the water stream itself as a "blast" media. Air washing is a method of cleaning that is essentially blast cleaning without any media. Both, like blasting, do an excellent job of aerosolizing particles from a surface, although air washing is often ineffective if the particles are adhered to the surface. Air washing, using a variety of delivery mechanisms, is often used as a component of air duct cleaning.
The aerosolization caused by blasting is not always a problem. For instance, if mold remediation is done outdoors or in an unenclosed space such as a structure that has not yet been closed in, the mold spores are able to just disperse. This assumes unprotected people are far enough away as to not be endangered and that contaminants are not being drawn into a building. Contents can be effectively and safely cleaned using blast methods either outside or in a true laminar airflow cleaning chamber.
There is another possible problem when blasting is used for mold remediation or for cleanup of other types of potentially hazardous particles. Most mold spores are 1–2 micrometers and larger. Thus they can be effectively removed from the air by HEPA filters (99.97% efficient at 0.3 micrometers) in respirators, vacuums and air filtration devices. It has been speculated that the harsh mechanical action of blasting may break apart mold spores into smaller particles, some of which are likely to be smaller than 0.3 micrometers. Since this kills the spore, there is no chance that it could cause an infection. However, the primary health effects of mold are allergic and perhaps toxic, not infectious. These effects are not reduced by being broken into very small particles. If this speculation is correct, these particles could pass right through HEPA filtration, potentially creating exposure and cross-contamination issues. Similar problems could perhaps arise when blasting is used to remove lead paint or other hazardous contaminants.
Most common media types, such as sand, are overly aggressive for the purposes mentioned above and cause unacceptable appearance or even structural damage to wood surfaces. Almost all media types, such as sodium bicarbonate ("soda"), leave a large amount of used media that must be cleaned up, in addition to the removed contaminants.
Some media, such as plastic chips and sponges with embedded abrasives, are designed to be recycled to make their use cost-effective. Recycling may be feasible for fire restoration uses, but according to IICRC S520 recycling of media used for mold remediation is not advisable, as it is not known whether present methods of recycling are adequate to remove mold contaminants effectively. So for mold remediation, these "recyclable" types of media should only be used once, which usually makes them prohibitively expensive.
Yet another potential disadvantage of blasting is that concentrated air pressure at the point of contact with the surface being cleaned has significant potential to force air, media and entrained contaminants through building penetrations into uncontaminated areas, even if the work area is kept under negative pressure.
Most blasting methods require a considerable investment in equipment and a significant amount of time to set up, which means that their use is a lot less efficient on smaller jobs.
Safety precautions necessary with any blast method may include eye, face, and respiratory protection. Most blast methods are very noisy, so hearing protection must be used.
The two most widely used media for restoration and mold remediation are soda and dry ice. We'll discuss these two individually.
Soda blasting is very effective at removing char, smoke residue and mold growth from framing without causing excessive damage to the wood. This is because the soda media is very soft.
Unfortunately, soda blasting produces a tremendous amount of very fine dust and leaves a lot of debris from the expended media that can be difficult and time-consuming to clean up. It has a fairly high production rate.
Dry ice blasting is certainly the most unusual of the blast media. It uses pellets or shavings of dry ice (solid CO2) as a blast media, which eliminates some of the disadvantages of other media.
Unlike other media, dry ice sublimates (goes from a solid to a gas without passing through a liquid phase) on impacting the surface being cleaned. When it goes from a solid to a gas, it tends to spread out across the surface of the material which causes a "shearing" effect that efficiently removes contaminants from the surface while causing little if any surface damage. For certain types of contaminants, such as oils and waxes, thermal shock also contributes to the cleaning effect. As the blast media becomes a gas and dissipates, only the contaminants removed from the surface are left to be cleaned up. Highly efficient removal of contaminants, minimal surface damage and no media residue to clean up seem to make dry ice blasting the ideal mold remediation method.
However, dry ice blasting has all the disadvantages of blasting in general, with the exception of a media waste stream, and it has some very serious and specific safety concerns. These can all be overcome, but you should plan before using this cleaning method.
Dry ice blasting methods vary considerably. Some use prepared pellets, while with others the machine generates its own pellets or flakes from blocks of dry ice. Dry ice media is significantly more expensive than most others, inherently inconvenient and potentially hazardous to handle. Some methods have a much higher production rate than others. The amount of dry ice used varies from as little as ¼ pound per minute to more than 10 pounds per minute.
The surface temperature of dry ice is -110°F, more than adequate to cause serious damage to human skin. Heavy gloves should always be worn when handling dry ice or its containers. Use tongs when handling dry ice blocks.
CO2 gas is 1.56 times as heavy as air. Since it fills up a space from below, it displaces oxygen and can potentially create a highly hazardous reduced-oxygen atmosphere. High concentrations of CO2 gas also have direct health effects, even if the oxygen level remains adequate. Containment may limit ventilation enough that excessive levels of CO2 may build up. Keep in mind that such a high-volume blast system adds a tremendous amount of air to the space (as propellant), in addition to the CO2. To maintain appropriate negative pressure differentials, you may have to significantly increase your volume of exhaust air.
We attempted to find some information on CO2 levels that are typically generated during dry ice blasting. We were unable to find any information that would allow us to report exposure levels. However, in small areas or confined spaces, especially one that is below grade, such as a crawlspace, these issues could become critical.
The OSHA Permissible Exposure Limit (PEL) for CO2 gas is 5000 ppm (0.5%) for an eight-hour time-weighted average; 40,000 ppm (4%) is the Immediately Dangerous to Life and Health (IDLH) concentration. Engineering controls, such as exhaust fans (HEPA filtered if the air being exhausted is or may be contaminated) at ground level may be able to keep it below these levels, if properly used.
Supplied-air or self-contained (SCBA) respirators must be used if engineering controls do not keep levels below the PEL. Continuous monitoring of CO2 levels, at least in confined spaces, is required to ensure worker safety.
According to the Canadian Centre for Occupational Health and Safety, "Exposure to 10% for 1.5 minutes has caused eye flickering, excitation and increased muscle activity and twitching. Concentrations greater than 10% have caused difficulty in breathing, impaired hearing, nausea, vomiting, a strangling sensation, sweating, stupor within several minutes and loss of consciousness within 15 minutes. Exposure to 30% has quickly resulted in unconsciousness and convulsions. Several deaths have been attributed to exposure to concentrations greater than 20%. Effects of CO2 can become more pronounced upon physical exertion, such as heavy work."
Blast methods of cleaning are extremely versatile and potentially have many uses in the restoration and remediation industries. However, inherent characteristics can cause undesired side effects if not adequately addressed.
— Jim Holland
