In the US, silica sand is prohibited from use in sandblasting applications due to the risk of silicosis.
Prohibition of Silica in Abrasive Blasting
Because of the high risk for silicosis in sandblasters and the difficulty in controlling exposures, the use of crystalline silica for blast cleaning operations was prohibited in Great Britain in 1950 [Factories Act 1949] and in other European countries in 1966 [ILO 1972]. In 1974, NIOSH recommended that silica sand (or other substances containing more than 1% free silica) be prohibited as abrasive blasting material and that less hazardous materials be used in blasting operations [NIOSH 1974b].
Current Exposure Limits
The current OSHA permissible exposure limit (PEL) for respirable crystalline silica (quartz) is 100 µg/m3 as an 8-hour time-weighted average (TWA) [29 CFR** 1910.1000]. The NIOSH recommended exposure limit (REL) for respirable crystalline silica is 50 µg/m3 as a TWA for up to 10 hours/day during a 40-hour workweek [NIOSH 1974b]. This REL is intended to prevent silicosis. However, evidence indicates that crystalline silica is a potential occupational carcinogen [NIOSH 1988a; IARC 1987; DHHS 1991], and NIOSH is reviewing the data on carcinogenicity.
I wouldn't bother using it anyways. It compacts the media and makes it weigh a ton. I highly reccommend using fine perlite instead. It lightens the mix, is much more available, and easier to work with. Please note, though, that inhaling it is still bad for you so watch out for that.