New fields being build may be natural grass or they may have a synthetic turf. There are many factors that may go in to determining which type of field is built in a given area including cost, climate, use, and others. However, increasingly, the chance of injury, is playing a more and more important role in determining which turf option will be placed.
There is not clear winner or safest option. Newer synthetic turf options, such as FieldTurf, are much better than older synthetic options such as AstroTurf. There have been a number of studies to try to determine if FieldTurf or natural turf is safer, but the jury is still out. A recent NFL study found more injuries on FieldTurf, including an 88% higher risk of ACL tears, and 32% higher risk of ankle sprains. Keep in mind, however, that this is comparing FieldTurf to NFL natural turfs. Most fields don't have NFL team grounds crews maintaining natural grass, and they often have more use. One reason FieldTurf may be advantageous, is that the maintenance and weather conditions have less of an impact on safety.
Research is ongoing, and newer turf surfaces are being developed that may be even safer than current options. It seems, at present, that a perfectly maintained natural grass surface may be ideal for preventing injuries, but if field maintenance is an issue, a synthetic surface may provide safer playing conditions.