Latex balloons may be inflated with either air or helium. Because latex is a porous material, the gas (helium or air) molecules pass through the surface, eventually causing the balloon to deflate or descend. When air-inflated, latex balloons stay inflated considerably longer than those inflated with helium because air molecules are larger and slower moving than helium molecules, so air doesn't escape as quickly as helium.
Helium-inflated balloons are affected by extreme temperatures and high altitudes. The rule for temperatures is to slightly under-inflate balloons when moving them from a cool environment to a warmer one (as the helium will expand), and slightly over-inflate them when moving them from a warm environment to a cooler one (as the helium will contract). For example, moving helium-filled balloons from an air-conditioned room to one that's not or to the outdoors on a warm or hot day will result in the balloon expanding. Test the balloons to determine proper inflation in each circumstance.
Altitude effects the float time of latex balloons as helium has less lift in higher altitudes. Balloon will not float as long in higher altitudes, so larger balloons should be used. For more precise results, helium inflate various sizes and track the times they float at your altitude.
Latex balloons can become covered with a velvet finish when atmospheric conditions change, such as in high humidity, in high ozone or when exposed to sunlight. This velvet finish is the result of oxidation, the first step in the bio-degradation (or natural breakdown) of the natural latex. To decrease the possibility of oxidation, cover balloons with a plastic bag, especially if they will be exposed for an length of time. When inflating large quantities of balloons for decorating, many professionals recommend inflating the balloons on-site to try and avoid oxidation when the balloons are transferred for the store to the site, especially when moving the balloons in and out of air-conditioned facilities.
To slow the process of oxidation, apply a product such as HI-FLOAT or Balloon Shine to the exterior of the balloon (following the manufacturers instructions).
Tying knots in balloon necks is the preferred method of sealing latex balloons as it provides the best seal, it doesn't add weight so the float times are not reduced, it is less time consuming and it is more cost effective.
To seal 36" and larger latex balloons, use Giant Quickie Clips or cable ties.