Sand Safety Surface
Play sand has become a popular impact absorbing surfacing material under playground equipment. It's non packing characteristics make it an attractive option. It also acts as a play toy in its own right making sandcastles, playing beach side games is always going to make sand a popular play surface with children.
When talking about sand for a safety surface you must choose play sand that is clearly marked. Using builders sand (one part of cement) that is treated to ensure it bonds together quickly and be very strong would be completely unsuitable in a play area. This would form a hard surface when mixed with water so the first time it rains you will find dangerous lumps forming.

Non-Packing
Non-packing is when the lose material doesn't compress and bond together under pressure. Play bark is a packing material as under pressure it will bind together. So with sand look for "washed" and designed specifically impact absorbing surfacing.
This will still give opportunities for creative and constructive play, but will still dry back to being a safe play surface. Sand really is a great surface for young children as it gives a great sensory experience and it's great for fine tuning physical movements. Think how many top athletes swear by running on the beach to improve fitness levels..
Safety Sand Drawbacks
When sand is wet, the sand surface becomes hard losing its effectiveness at absorbing impacts. Children will have to wait for dry sand before it becomes a suitable impact absorbing surface. This means that drainage must be adequate otherwise the area becomes saturated and stagnates. This situation also occurs in humid conditions where it can also compromise sand.
Whilst sand is great for kids developing movement, children in wheelchairs or other mobility devices sand is absolute nightmare, making the play equipment unreachable for these children who would benefit from play equipment.
Sand gets everywhere, and this will lead to it being swallowed getting into eyes and ears all cause problems beyond tracking sand back into the house. On warm days sand can become very light, good for impact absorption not so great in windy conditions.
Sand is also a perfect environment for some insects and resembles a litter tray, not to mention it disguises any sharp objects that are just below the surface. Sand requires regular maintenance, Racking every day to avoid any build up and remove foreign objects.
Safety Sand Advantages
One of the biggest draws of play sand is that it has far lower set up cost than something like wet pour, its easily available and requires competitively less equipment and skill set to put up.
Sand is also not flammable, setting sand alight is virtually impossible unless you're in a lab with some extremely dangerous chemicals that would likely raise a few eyebrows. That being said scorched sand would have to be replaced. Sand will not support microbial growth so whilst it will make a nice home for some insects its does prevent this.
All being said in good conditions it will provide excellent absorption qualities and at a fraction of the initial cost of other types. Most difficulties with sand can be limited by regular maintenance, this is a matter of knowing if you have spare time to do this for until you no longer want a play area.
Sand is also play equipment in it's own right and this sensory play is well documented to be good for young children, with care taken to make sure that an even spread is maintained across the safety area there is no reason why it could not be enjoyed.

Sand Final Thoughts
Well maintained sand is an excellent safety surface easy to install natural and washed sand is perfectly safe and it double as play equipment in it's own right. However, to often its left to become compressed together despite being a non-packing surface it compacts tightly and this affects it ability to act as a impact absorbing surface.