Some newer models claim to be better than the first models because of “washable” filters. Generally, washable true HEPA filters are expensive. Some manufacturers claim filter therapure air purifier standards such as “HEPA 4”, without explaining the meaning behind them. This refers to their MERV rating. These ratings are used to rate the ability of an air cleaner filter to remove dust from the air as it passes through the filter. MERV is a standard used to measure the overall efficiency of a filter. The MERV scale ranges from 1 to 20, and measures a filter’s ability to remove particles from 10 to 0.3 micrometre in size. Filters with higher ratings not only remove more particles from the air, they also remove smaller particles.