The county council has warned vulnerable people to not "engage in strenuous activity" while the county is still affected by poor air quality.
Teresa Heritage, Cabinet Member for public health at Hertfordshire County Council, said: "We've seen a rise in pollution levels due to a mix of fine sand in the atmosphere from the Sahara, prevailing weather conditions and particles from traffic and other activity. It's particularly high in parts of Eastern England and is forecast to be here until Friday when it gets much fresher and we expect some rain."
"While there is no major adverse health impact from the dust long term, and many people will not notice much difference other than haze, people who have asthma or get breathless are particularly vulnerable to the dust causing some short term avoidable problems, which could be serious in some cases.
"If you have asthma, a heart condition, are very overweight or have another long-standing health problem which may leave you breathless, it is particularly important that you do not engage in strenuous activity like running or cycling until the pollution clears and check the conditions on future days."
Defra has said the conditions are expected to improve by the end of today.