Catching kids when they fall: guidelines to choosing a playground surface - Cover Story

Parks & Recreation, April, 1997 by Walter Henderson

The importance of proper resilient surfacing in playground safety has become widely recognized in recent years. Indeed, since the US Consumer Product Safety Commission (CPSC) has estimated that up to 60% of playground injuries may be caused by falls to the surface, and since injuries from falls to the surface tend to be among the most serious playground injuries, it can be argued that providing a shockabsorbing surface under playground equipment may be the single most important action that can be taken to improve a playground's safety. While the decision to provide resilient playground surfacing is fairly straight-forward, the choice of which surface to use is much more complex. There are literally dozens of playground surfaces available to the playground administrator today. Furthermore, in addition to shock absorbency, many other factors--such as accessibility, maintenance and cost--must be considered. This article will survey the most commonly used playground surfaces and their performance in the key areas of shock absorbency, accessibility, cost, and maintenance.

Types of Resilient Surfacing

Resilient playground surfaces commonly used in the United States include inorganic loose fill materials (sand and gravel) organic loose fill materials, wood mulch, bark mulch and wood chips, wood fiber, rubber mats, and troweled-in-place rubber surfacing. All of these categories will be discussed in this article.

Other, less frequently used surfaces include rubber particles or shredded rubber; closed cell foam overlaid with rubber sheeting; and rubber foam or mats topped with artificial turf or indoor/ outdoor carpet. These systems, however, will not be discussed in this article.

Shock Absorbency

Shock absorbency is the most critical consideration in choosing a playground surface. Surfacing that does not offer adequate shock absorbency should not be used under playground equipment no matter what other advantages it may offer.

In order to provide a means of assessing s hock absorbency, the American Society for Testing and Materials (ASTM) has specified a test procedure to measure the deceleration imparted to an instrumented headform when it strikes a sample of surfacing material. ASTM and the CPSC recommend that the peak deceleration at impact not exceed 200 times the acceleration due to gravity (200 Gmax) and that HIC (Head Injury Criteria) be less than 1000, a measure of deceleration over the time which the headform decelerates to a halt. According to the CPSC, if these two measures are met, a life-threatening head injury would not be expected to occur.

The critical height of a material, a term used by the CPSC and others, is the maximum height at which the peak deceleration is less than 200 Gmax and HIC is less than 1000. The CPSC recommends that resilient surfacing around a piece of playground equipment have a critical height equal to or greater than that of the highest accessible part of the equipment.

This recommendation rules out the use of asphalt and concrete, which exceed 200 Gmax at a critical height of only two inches. Soil, hard-packed dirt, grass, and turf are also not recommended because their shock absorbency may be poor due to wear, moisture absorption, and packing.

Suppliers of the most common loose fill materials rarely provide shock absorbency test data, since these materials are most commonly used for construction or landscaping purposes, not playground surfacing. Therefore the CPSC tested several widely used materials for shock absorbency and published the results in The Handbook for Public Playground Safety (see Fig 1).

FIGURE 1. CRITICAL HEIGHTS (IN FEET) OF COMMON LOOSE MATERIALS

                              Uncompressed Depth   Compressed Depth

Material                     6 inch   9 inch   12 inch   9 inch

Wood Mulch                      7       10        11       10

Double Shredded Bark Mulch      6       10        11        7

Uniform Wood Chips              6        7       >12        6

Fine Sand                       5        5         9        5

Coarse Sand                     5        5         6        4

Fine Gravel                     6        7        10        6

Medium Gravel                   5        5         6        5

Source: The Handbook For Public Playground Safety, US Consumer Product Safety Commission

A depth of six to 12 inches of most of these materials provides critical heights of five to 10 feet. This range covers the highest accessible part of most playground equipment sold in the US today. Loose fill materials lose some shock absorbency under compression, which is likely to occur on most playgrounds from user foot traffic.

While the critical height information is extremely useful, some questions about the shock absorbency of loose fill materials remain unanswered. The published CPSC tests were conducted at ambient temperatures, and thus provide no information about performance at high and low temperatures. Additionally, users of loose fill materials must ensure that the materials that they use are similar in composition to those tested by the CPSC to be certain of equivalent shock absorbency. Finally, some other possible loose fill materials like bark nuggets, shredded rubber and ground rubber have not been tested.

Some of these questions may be answered by The National Program for Playground Safety at the University of Northern Iowa, which is being funded by the Centers for Disease Control and Prevention (CDC) of the United States Public Health Service (See Hudson, Thompson and Mack, p.69). This program includes research on the performance of most common types of playground surfaces, according to ASTM 1292, which requires testing at 120 [degrees] F and 30 [degrees] F in addition to testing at ambient temperatures. The results of the this program may provide information about a wider variety of resilient surfaces at varied temperatures. Results of this testing are expected to be available later this year. More information on this testing, and other aspects of the National Program for Playground Safety, may be obtained by calling 800-554-7529.