Fresh salads do not cause gas. However pre washed salads that are available in cartons in wholesale store can cause gas. Pre washed salads are treated in chlorine and the bags contain a variety of gases, so that the salads can retain their freshness. Preservatives may also be added in pre washed salads to increase their shelf life. Fresh vegetable and fruit salad may cause gas, if you have the habit of eating more processed foods. However, this will reduce after a few days.