Head Support Cushions: If the manufacturer incorporates a head support cushion with the restraint system, then the manufacturer has tested it for that model and make of restraint. Should a head support cushion that is purchased separately be used, then it is possible that the slots in the head support cushion may not line up with the slots in the restraint for the harness system. By re-routing the harness system to accommodate the aftermarket head support cushion, slack may be induced into the harness system, which may increase the likelihood of ejection from the car seat in the event of a crash.
Additional Padding Behind the Child: Any additional padding behind the child can induce both slack in the harness and additional compressibility. The Standard, which regulates children's restraint systems, only allows for a certain amount of compressibility in the foam and material used. By increasing this amount, during a collision the additional foam/material can compress to the point that the harness system becomes very loose and therefore no longer is capable of restraining the child.
Padded Car Seat Bags: Child seat manufacturers state in their instructions not to use bulky clothing and never add anything between the shell of the restraint and the child. The padded car seat bag can re-route the harness system and add slack and increase compressibility. Check with the car seat manufacturer before using and ensure that the product does not compromise the harness routing path.
Mobiles: Mobiles are becoming common as they keep the child occupied during road trips. These products normally hang from the handle of a rear-facing infant restraint. If these mobiles are constructed of a hard plastic, during a collision they could injure the child. Normally the car seat manufacturer recommends that the handle of the restraint be in the down position while traveling in a vehicle. Check your car seat instructions before using.
Winter Clothing: Bulky snowsuits can affect the harness with respect to additional compressibility. In addition, many snowsuits are made of very slippery material. This can affect the harness system should the chest clip of the restraint not be used properly. When using bulky winter clothing ensure that the harness system is tight, compressing the material to ensure a snug fit. Check with the car seat manufacturer for alternative methods of clothing during the winter.
Seat Belt Adjusters: Third-party aftermarket seat belt adjusters have become very common. Many of these products pull the shoulder portion and the lap portion of the seat belt together. This tends to pull the shoulder portion of the seat belt off the face of the child but normally pulls the lap portion of the seat belt up onto the soft abdominal area. In the event of collision, the location of the lap belt can cause serious injury or death to the occupant. There are other types of adjusters available, all of which are designed to pull the shoulder belt away from the face of a child and all of which change the configuration of the lap belt. If an aftermarket product is required to re-configure the seat belt assembly to sit over the shoulder and low on the hips, then the child should probably be in a booster cushion.