The most common source of lead poisoning is lead-contaminated dust (LCD). This dust can be so fine that it cannot be seen by the naked eye. Dusts can have thousands of parts per million lead by weight. The high concentrations of lead in LCD means that even tiny quantities of dust exposure can result in a very high lead intake. The current government standards for safe levels of lead-contaminated dust in housing are 100 µg/ft2 on floors; 500 µg/ft2 on window sills; and 800 µg/ft2 on window wells. Some experts believe these levels are too high and should be lowered.