Not good enough at absorbing impact, ergo, deemed unsafe[1]. Last I recall, the recommendation was to replace it with wood chips, but I think even that's been superseded to be rubber chips instead, which also happen to be considerably more expensive.
To my eye, I deem spurious any science that concludes their findings with 'grass is unsafe', but at the same time I must acknowledge that sure, many of the things we did as kids were dangerous, and just because it was normative to me doesn't mean that it should be normative -- in the same way that kids no longer ride in the backs of pickup trucks, or try to loop around the swingset.
That said, it's still pretty hard to get over the fact that the ground is considered actively harmful enough that we would close down playgrounds in response.
Perhaps you mistyped? The conclusion in the paper you linked to was the opposite:
Granitic sand playground surfaces reduce the risk of arm fractures from playground falls when compared with engineered wood fibre surfaces. Upgrading playground surfacing standards to reflect this information will prevent arm fractures.
Probably I mis-remembered and my slapdash attempt at finding a source was in error (which makes twice in two days for me, embarrassingly.)
I remember of the studies perhaps better than I remember the studies, but the gist is that there's a hierarchy of playground surfaces. I thought it was:
grass < sand < wood < rubber
but I suppose it's more like
grass < wood < sand < rubber
Regardless, sand does appear to be considered acceptable by CPSC guidelines[1], in which they recommend ATSM F1292 tested wood fibers, pea gravel, sand, rubber mulch, non-CCA-treated wood mulch, and recommend against asphalt, concrete, carpet not tested to ATSM F1292, dirt, grass, and CCA-treated wood mulch.
Apologies for the error, and thanks for the correction.
To my eye, I deem spurious any science that concludes their findings with 'grass is unsafe', but at the same time I must acknowledge that sure, many of the things we did as kids were dangerous, and just because it was normative to me doesn't mean that it should be normative -- in the same way that kids no longer ride in the backs of pickup trucks, or try to loop around the swingset.
That said, it's still pretty hard to get over the fact that the ground is considered actively harmful enough that we would close down playgrounds in response.
[1] - http://journals.plos.org/plosmedicine/article?id=10.1371/jou...