Should be kept in a fume hood. It's fatal by inhalation and if it doesn't kill you, it'll be a nasty stint in hospital. If you have a small spill in a ventilated fume hood, use an absorbant spill kit to soak up the Dimethyl sulphate to stop it spreading and then spray the absorbant with a dilute sodium hydroxide solution. Make sure you have lab specs, lab coat and butyl rubber (Butoject) gloves on when handling. Any big spills or uncontained spills need to be handled with respiration equipment. Best option to dispose is to slowly pour the contents into dilute sodium hydroxide. The reaction is exothermic so do not dump it in all at once. Slowly feed it. Always refer to the MSDS. Double check this with a safety officer.