Engineered wood fiber (EWF) is often referred to as wood mulch, yet it's really not the same thing at all, because EWF is sometimes heat treated, then tumbled to get rid of the sharper edges to make it more smooth and less likely to result in spinters, yet it does continue to deteriorate rapidly over time and requires frequent churning or re-blending.
EWF is about as common as playground sand, yet unfortunately is often poorly maintained, and often deteriorates quicker than playground sand.
Cost: Between $25 to $40 per square foot over 25 years, if installed outdoors and maintained and/or replaced as recommended by the NRC (National Resource Center for Health and Safety in Child Care); Initially around $2.50 per square foot for a 12" depth; Ongoing about $2.50+ per square foot every two and one-half years if replaced, or $1.50 per square foot to churn or loosen up the material and replenish some every year or so to maintain the 12" depth
Safety: About the same as playground sand and nearly as common safety surfacing with pretty good HIC scores for lower fall heights, not really safe for maximum fall heights of over twelve feet
Maintenance: The primary maintenance involves re-blending and mixing up the EWF to redistribute the deteriorating smaller particulates to keep it from becoming a fine dust, and this can be expensive for the labor involved for periodic annual mixing in addition to re-topping or replacement as recommended every two years
Hazards: It's no surprise that wood attracts certain pests and also absorbs moisture, which can lead to microbes, often in the form of fibrous fungus that effectively turns it into a solid mass and breaks down the EWF quicker and eliminates the impact attentuation, in addition to slime mold (also called dog vomit mold due to what it looks like, often appearing white, yellow, orange or black in color), and it can attract nesting pests and vermin such as termites, mice and rats, and others.
Liabilities: Due to the commonly rapid deterioration of EWF and lack of proper maintenance, the impact attentuation can quickly become limited due to microbial growth of mold, mildew, fungus and/or bacteria, and certain pests like vermin carry with them a host of pathogens that can cause serious illness
Engineered wood fiber (EWF) is really the only form of wood product that ought to be considered for playground use, if choosing wood, because raw chunks of wood or bark can contain potentially harmful sharp edges, splinters, microbes and pests, to name a few. Yeah, throwing those tree branches into a chipper to use for a playground might seem inexpensive, until the injuries and claims start to arise. Initially, the required twelve inch depth it needs to be installed at provides fairly decent injury protection as a playground safety surface, because it disperses slightly, as well as compresses, to a lesser degree to that of rubber mulch.
The primary issues with wood products are related to them being organic and they continue to break down over time, into what ultimately becomes a fine dust that retains moisture better, resulting in growing microbes from anything it's been exposed to. Wood products are also what attracts vermin such as rats or mice to nest within, which isn't really good to have within a playground (a snake was living in one EWF playground, because there were so many rodents, it didn't need to go elsewhere for food!).
Wood products also attract other forms of microbes, due to the absorption of moisture, including mold, mildew, fungus and bacteria, and can also absorb liquid pesticides and fertilizers. Wood fungus can effectively knit together the wood particles, creating a more solid mass that can eliminate most of the fall injury protection. While this is good for healthy trees and plants, it's not so good for any playground surface. Wood products also attract different forms of mold, such as slime mold that is also referred to as "dog vomit" mold, because of it's appearance, normally white, yellow, orange or black in color. While it isn't harmful to humans, many are alarmed by its appearance, and it can attack and cause harm to sensitive plant roots.
The proper maintenance for wood products on playgrounds is churning the material on a regular basis, although this also results in allowing the finer detritus particulates to settle at the bottom, which is a normal problem with using wood. When wood deteriorates, more has to be added to meet the necessary twelve inch requirement. The issue here is you really need to get rid of all of the wood dust at the bottom, because that depth doesn't really count, and there's no good way to get rid of it without completely removing and replacing the entire contents.
Add all of this up, and wood products start to appear less safe, requiring a whole lot more attention to regular inspection and maintenance than most communities are willing to invest. Ultimately, many playgrounds with wood for safety surfacing deteriorate rather quickly and don't receive the upkeep they require to maintain their relative fall safety, as well as dealing with any unwanted encroachment or infestation by mold, mildew, fungus, bacteria, termites, vermin & other pests (and snakes!).